Fix both socket-[un]constrained GRES allocation issues.

Do not assume that these sock_gres_t pointers always exist:
bits_by_sock
bits_by_sock[s]

If they don't, that means there are no current iteration socket `s`
constrained GRES and so the logic shouldn't allocate the current
iteration GRES `g`.

Analogously, do not assume that bits_any_sock sock_gres_t member pointer
is always valid. If it isn't, it means there are no socket-unconstrained
GRES available to satisfy the job request, so the logic should not
allocate the current iteration GRES `g`.

Otherwise, job/node struct members holding GRES allocation information
would end up being incorrect, leading to improper allocations and also
leading to errors logged in slurmctld log at deallocation time like:

error: gres/gpu: job <X> dealloc node <Y> GRES count underflow (0 < 1)

Bug 7827