1 exit path for returning ptr in Perl_safesysmalloc and Perl_safesysrealloc

commit 6edcbed640 goto-ed around an initialization and was partially
reverted in commit c62df97fd6 . This patch restores the intention of
commit 6edcbed640 by having only 1 exit path that will be returning
a pointer (and not croaking).
op.c: use GV_NOTQUAL in newATTRSUB_x

If we are scanning for the package separator ourselves, then we can
notify gv_fetchsv that we found none, so it doesn’t have to scan
again.