Stop &xsub and goto &xsub from crashing on undef *_

Stop &xsub and goto &xsub from crashing on undef *_

$ perl -e 'undef *_; &Internals::V'
Segmentation fault: 11
$ perl -e 'sub { undef *_; goto &Internals::V }->()'
$ perl5.18.1 -e 'sub { undef *_; goto &Internals::V }->()'
Segmentation fault: 11

The goto case is actually a regression from 5.16 (049bd5ffd62), as
goto used to ignore changes to *_.  (Fixing one bug uncovers another.)

We shouldn’t assume that GvAV(PL_defgv) (*_{ARRAY}) gives us anything.

While we’re at it, since we have to add extra checks anyway, use them
to speed up empty @_ in goto (by checking items, rather than arg).