GenerateStructDefinition(): Reset vars["opt_comma"] when processing multiple oneofs

This commit fixes #251, which causes incorrect code to be generated when
there are multiple oneofs in the same message.

In #221, we added code to force int-sizing for oneof enums, but we only
set vars["opt_comma"] initially, before entering the loop over the
message's oneofs. This caused commas to be omitted when generating the
enums for subsequent oneofs after the first oneof.

This commit resets vars["opt_comma"] every time through the loop that
generates the enum declarations for the message's oneofs.

protoc-c/c_message.cc

@@ -148,9 +148,10 @@ GenerateStructDefinition(io::Printer* printer) {
   }
 
   // Generate the case enums for unions
-  vars["opt_comma"] = ",";
   for (int i = 0; i < descriptor_->oneof_decl_count(); i++) {
     const OneofDescriptor *oneof = descriptor_->oneof_decl(i);
+    vars["opt_comma"] = ",";
+
     vars["oneofname"] = FullNameToUpper(oneof->name());
     vars["foneofname"] = FullNameToC(oneof->full_name());