[perl #125541] Fix crash with %::=(); J->${\"::"}

[perl #125541] Fix crash with %::=(); J->${\"::"}

gv_stashpvn does not expect gv_fetchpv to return something that is
not a GV.  If someone has blown away the stash with %::=(), then the
$::{"main::"} entry no longer exists, but gv_fetchpv expects it to be
there.  This patch just makes this case fail somewhat gracefully
instead of crashing:

$  ./miniperl -e '%::=(); J->${\"::"}'
Can't locate object method "" via package "" (perhaps you forgot to load ""?) at -e line 1.

If someone does %::=(), nothing more reasonable should be expected.
At least it does not crash now.