Merge pull request #3064 from randomguy3/offset-type

Fix offset type macro to match the tables it is used in

src/google/protobuf/generated_message_util.h

@@ -100,7 +100,7 @@ namespace internal {
 // choose 16 rather than some other number just in case the compiler would
 // be confused by an unaligned pointer.
 #define GOOGLE_PROTOBUF_GENERATED_MESSAGE_FIELD_OFFSET(TYPE, FIELD)  \
-  static_cast<int>(                                                  \
+  static_cast< ::google::protobuf::uint32>(                           \
       reinterpret_cast<const char*>(                                 \
           &reinterpret_cast<const TYPE*>(16)->FIELD) -               \
       reinterpret_cast<const char*>(16))