Fix ruby gc_test in ruby 2.4 (#4011)

* Fix ruby gc_test in ruby 2.4

* Initialize global variables to Qnil.

ruby/ext/google/protobuf_c/defs.c

@@ -76,7 +76,7 @@ static upb_enumdef* check_enum_notfrozen(const upb_enumdef* def) {
 // -----------------------------------------------------------------------------
 
 #define DEFINE_CLASS(name, string_name)                             \
-    VALUE c ## name;                                                \
+    VALUE c ## name = Qnil;                                         \
     const rb_data_type_t _ ## name ## _type = {                     \
       string_name,                                                  \
       { name ## _mark, name ## _free, NULL },                       \
@@ -126,11 +126,11 @@ void DescriptorPool_register(VALUE module) {
   rb_define_method(klass, "lookup", DescriptorPool_lookup, 1);
   rb_define_singleton_method(klass, "generated_pool",
                              DescriptorPool_generated_pool, 0);
-  cDescriptorPool = klass;
   rb_gc_register_address(&cDescriptorPool);
+  cDescriptorPool = klass;
 
-  generated_pool = rb_class_new_instance(0, NULL, klass);
   rb_gc_register_address(&generated_pool);
+  generated_pool = rb_class_new_instance(0, NULL, klass);
 }
 
 static void add_descriptor_to_pool(DescriptorPool* self,
@@ -299,8 +299,8 @@ void Descriptor_register(VALUE module) {
   rb_define_method(klass, "name", Descriptor_name, 0);
   rb_define_method(klass, "name=", Descriptor_name_set, 1);
   rb_include_module(klass, rb_mEnumerable);
-  cDescriptor = klass;
   rb_gc_register_address(&cDescriptor);
+  cDescriptor = klass;
 }
 
 /*
@@ -518,8 +518,8 @@ void FieldDescriptor_register(VALUE module) {
   rb_define_method(klass, "subtype", FieldDescriptor_subtype, 0);
   rb_define_method(klass, "get", FieldDescriptor_get, 1);
   rb_define_method(klass, "set", FieldDescriptor_set, 2);
-  cFieldDescriptor = klass;
   rb_gc_register_address(&cFieldDescriptor);
+  cFieldDescriptor = klass;
 }
 
 /*
@@ -916,8 +916,8 @@ void OneofDescriptor_register(VALUE module) {
   rb_define_method(klass, "add_field", OneofDescriptor_add_field, 1);
   rb_define_method(klass, "each", OneofDescriptor_each, 0);
   rb_include_module(klass, rb_mEnumerable);
-  cOneofDescriptor = klass;
   rb_gc_register_address(&cOneofDescriptor);
+  cOneofDescriptor = klass;
 }
 
 /*
@@ -1037,8 +1037,8 @@ void EnumDescriptor_register(VALUE module) {
   rb_define_method(klass, "each", EnumDescriptor_each, 0);
   rb_define_method(klass, "enummodule", EnumDescriptor_enummodule, 0);
   rb_include_module(klass, rb_mEnumerable);
-  cEnumDescriptor = klass;
   rb_gc_register_address(&cEnumDescriptor);
+  cEnumDescriptor = klass;
 }
 
 /*
@@ -1202,8 +1202,8 @@ void MessageBuilderContext_register(VALUE module) {
   rb_define_method(klass, "repeated", MessageBuilderContext_repeated, -1);
   rb_define_method(klass, "map", MessageBuilderContext_map, -1);
   rb_define_method(klass, "oneof", MessageBuilderContext_oneof, 1);
-  cMessageBuilderContext = klass;
   rb_gc_register_address(&cMessageBuilderContext);
+  cMessageBuilderContext = klass;
 }
 
 /*
@@ -1491,8 +1491,8 @@ void OneofBuilderContext_register(VALUE module) {
   rb_define_method(klass, "initialize",
                    OneofBuilderContext_initialize, 2);
   rb_define_method(klass, "optional", OneofBuilderContext_optional, -1);
-  cOneofBuilderContext = klass;
   rb_gc_register_address(&cOneofBuilderContext);
+  cOneofBuilderContext = klass;
 }
 
 /*
@@ -1569,8 +1569,8 @@ void EnumBuilderContext_register(VALUE module) {
   rb_define_method(klass, "initialize",
                    EnumBuilderContext_initialize, 1);
   rb_define_method(klass, "value", EnumBuilderContext_value, 2);
-  cEnumBuilderContext = klass;
   rb_gc_register_address(&cEnumBuilderContext);
+  cEnumBuilderContext = klass;
 }
 
 /*
@@ -1645,8 +1645,8 @@ void Builder_register(VALUE module) {
   rb_define_method(klass, "add_enum", Builder_add_enum, 1);
   rb_define_method(klass, "initialize", Builder_initialize, 0);
   rb_define_method(klass, "finalize_to_pool", Builder_finalize_to_pool, 1);
-  cBuilder = klass;
   rb_gc_register_address(&cBuilder);
+  cBuilder = klass;
 }
 
 /*

ruby/ext/google/protobuf_c/map.c

@@ -825,8 +825,8 @@ VALUE Map_iter_value(Map_iter* iter) {
 void Map_register(VALUE module) {
   VALUE klass = rb_define_class_under(module, "Map", rb_cObject);
   rb_define_alloc_func(klass, Map_alloc);
-  cMap = klass;
   rb_gc_register_address(&cMap);
+  cMap = klass;
 
   rb_define_method(klass, "initialize", Map_init, -1);
   rb_define_method(klass, "each", Map_each, 0);

ruby/ext/google/protobuf_c/repeated_field.c

@@ -626,8 +626,8 @@ void RepeatedField_register(VALUE module) {
   VALUE klass = rb_define_class_under(
       module, "RepeatedField", rb_cObject);
   rb_define_alloc_func(klass, RepeatedField_alloc);
-  cRepeatedField = klass;
   rb_gc_register_address(&cRepeatedField);
+  cRepeatedField = klass;
 
   rb_define_method(klass, "initialize",
                    RepeatedField_init, -1);