BUILD: ssl_sock: remove build warnings on potential null-derefs

BUILD: ssl_sock: remove build warnings on potential null-derefs

When building with -Wnull-dereferences, gcc sees some cases where a
pointer is dereferenced after a check may set it to null. While all of
these are already guarded by either a preliminary test or the code's
construction (eg: listeners code being called only on listeners), it
cannot be blamed for not "seeing" this, so better use the unguarded
calls everywhere this happens, particularly after checks. This is a
step towards building with -Wextra.