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).

1 files changed, 1 insertions, 1 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 2598ef0f58..03ce10263d 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -2714,7 +2714,7 @@ try_autoload:
 	    !CvLVALUE(cv))
 	    DIE(aTHX_ "Can't modify non-lvalue subroutine call");
 
-	if (!hasargs) {
+	if (!hasargs && GvAV(PL_defgv)) {
 	    /* Need to copy @_ to stack. Alternative may be to
 	     * switch stack to @_, and copy return values
 	     * back. This would allow popping @_ in XSUB, e.g.. XXXX */