Ruby: Only add RepeatedField instance to cache once fully initialized

Fix: protocolbuffers#11968

Since calling a method on WeakMap allow the RubyVM to switch threads,
adding a non-fully initialized object into the cache allow another thread
from using it.

Note: with this fix, the other two thread may create their own wrapper
for the same underlying Array. We don't know if it's a big deal or not.

Co-Authored-By: Peter Zhu <peter@peterzhu.ca>

ruby/ext/google/protobuf_c/repeated_field.c

@@ -87,14 +87,14 @@ VALUE RepeatedField_GetRubyWrapper(upb_Array* array, TypeInfo type_info,
   if (val == Qnil) {
     val = RepeatedField_alloc(cRepeatedField);
     RepeatedField* self;
-    ObjectCache_Add(array, val);
     TypedData_Get_Struct(val, RepeatedField, &RepeatedField_type, self);
     self->array = array;
     self->arena = arena;
     self->type_info = type_info;
     if (self->type_info.type == kUpb_CType_Message) {
       self->type_class = Descriptor_DefToClass(type_info.def.msgdef);
     }
+    ObjectCache_Add(array, val);
   }
 
   PBRUBY_ASSERT(ruby_to_RepeatedField(val)->type_info.type == type_info.type);