Ignore incomplete extensions when building a FileDescriptor

FileDescriptor construction uses an extension registry including extensions from imports. If these were created using an older version of protoc, the FieldDescriptor.Extension property may be null; we ignore such extensions rather than failing.

csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs

@@ -245,6 +245,9 @@ namespace Google.Protobuf.Reflection
 
         /// <summary>
         /// Unmodifiable list of top-level extensions declared in this file.
+        /// Note that some extensions may be incomplete (FieldDescriptor.Extension may be null)
+        /// if this descriptor was generated using a version of protoc that did not fully
+        /// support extensions in C#.
         /// </summary>
         public ExtensionCollection Extensions { get; }
 
@@ -419,6 +422,7 @@ namespace Google.Protobuf.Reflection
         {
             return descriptor.Extensions.UnorderedExtensions
                 .Select(s => s.Extension)
+                .Where(e => e != null)
                 .Concat(descriptor.Dependencies.Concat(descriptor.PublicDependencies).SelectMany(GetAllDependedExtensions))
                 .Concat(descriptor.MessageTypes.SelectMany(GetAllDependedExtensionsFromMessage));
         }
@@ -427,6 +431,7 @@ namespace Google.Protobuf.Reflection
         {
             return descriptor.Extensions.UnorderedExtensions
                 .Select(s => s.Extension)
+                .Where(e => e != null)
                 .Concat(descriptor.NestedTypes.SelectMany(GetAllDependedExtensionsFromMessage));
         }
 

csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs

@@ -198,7 +198,10 @@ namespace Google.Protobuf.Reflection
         public FieldCollection Fields { get; }
 
         /// <summary>
-        /// An unmodifiable list of extensions defined in this message's scrope
+        /// An unmodifiable list of extensions defined in this message's scope.
+        /// Note that some extensions may be incomplete (FieldDescriptor.Extension may be null)
+        /// if they are declared in a file generated using a version of protoc that did not fully
+        /// support extensions in C#.
         /// </summary>
         public ExtensionCollection Extensions { get; }