G++ 9.0 maybe-uninitialized warning

[martin-cs]

I think this may be an improvement in GCC's maybe-uninitialized check but it means that CPROVER can't be built using GCC 9.0:

Complexity_limiter.cpp:101:60 error: '*((void*)& loop_to_blacklist +8)'  may be used uninitialized in this function [-Werror=maybe-uninitialized]

@JohnDumbell : I think this is your code and a glance does suggest it is possible that loops_to_blacklist does not get modified before it is returned.

[tautschnig]

This was fixed in f022757 (part of #5355).