perlop tweak

Consider this a follow-up to fc6933476.
Teach dump.c:debop about CV refs in stashes

This is what -Dt was giving:

$ ./miniperl -Dt -e 'sub foo{} &foo'

EXECUTING...

(-e:0)enter
(-e:0)nextstate
(-e:1)pushmark
(-e:1)gvAssertion failed: (isGV_with_GP(_gvstash)), function Perl_gv_fullname4, file gv.c, line 2341.
Abort trap: 6