STRUCT_MEMBER_PTR macro
Return field in a `ProtobufCMessage` based on offset. Take a pointer to a `ProtobufCMessage` and find the field at the offset. Cast it to a pointer to the passed type.
Syntax
#define STRUCT_MEMBER_PTR(member_type, struct_p, struct_offset) \
((member_type *) STRUCT_MEMBER_P((struct_p), (struct_offset)))