`SOCKET_OFFSETOF` was added in the initial commit {rBec51175f1fd6c91d5}
when `offsetof` [1] was not supported well enough. GCC and LLVM
support it since C++17.
Other two changes: type and size check can be done without creating
an invalid address too.
[1] https://cppreference.com/w/cpp/types/offsetof
Reviewed By: campbellbarton, brecht
Maniphest Tasks: T81100
Differential Revision: https://developer.blender.org/D9042
c1cdde04e2