Fix two bugs when calling &xsub when @_ has holes

This fixes #132729 in the particular instance where an XSUB is
called via ampersand syntax when @_ has ‘holes’, or nonexistent ele-
ments, as in:

    @_ = ();
    $_[1] = 1;
    &xsub;

This means that if the XSUB or something it calls unshifts @_, the
first argument passed to the XSUB will now refer to $_[1], not $_[0];
i.e., as of this commit it is correctly shifted over.  Previously, a
‘defelem’ was used, which is a magical scalar that remembers its index
in the array, independent of whether the array was shifted.

In addition, the old code failed to mortalize the defelem, so this
commit fixes a memory leak with the new ‘non-elem’ mechanism (a spe-
cially-marked element stored in the array itself).

1 files changed, 1 insertions, 1 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 9135e5d2d3..24c86e46fe 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -5215,7 +5215,7 @@ PP(pp_entersub)
 		    else sv = AvARRAY(av)[i];
 		    if (sv) SP[i+1] = sv;
 		    else {
-			SP[i+1] = newSVavdefelem(av, i, 1);
+			SP[i+1] = av_nonelem(av, i);
 		    }
 		}
 		SP += items;