Fix "sometimes-uninitialized" warning on Windows Clang.

src/google/protobuf/compiler/subprocess.cc

@@ -171,7 +171,7 @@ bool Subprocess::Communicate(const Message& input, Message* output,
     DWORD wait_result =
         WaitForMultipleObjects(handle_count, handles, FALSE, INFINITE);
 
-    HANDLE signaled_handle;
+    HANDLE signaled_handle = NULL;
     if (wait_result >= WAIT_OBJECT_0 &&
         wait_result < WAIT_OBJECT_0 + handle_count) {
       signaled_handle = handles[wait_result - WAIT_OBJECT_0];