when (re)creating @_, zero its elements

When a sub's @_ gets abandoned (e.g. when 'goto &bar' "donates" the
current @_ to the new sub), a new AV is created at pad[0] for the old
sub. Perl alloc()'s the AvARRAY() buffer for the new AV, but didn't zero
it. This didn't used to be a problem, because @_ AVs are created
!AvREAL(), so the first thing perl did when someone tries to modify @_,
is to call av_reify() on it.  And it just so happens that av_reify()
zeros out all the unused slots on the array first. So code like

    $_[1] = 1;
    do_something_with($_[0])

was fine.

However, on PERL_RC_STACK builds, @_ is AvREAL() by default now, so
av_reify() doesn't get called, so AvARRAY() can sometimes contain random
wild pointers.

The fix is simple: zero AvARRAY() when creating it.

This was showing as a failure in Test::Trap

pp_hot.c

@@ -5493,7 +5493,7 @@ Perl_clear_defarray(pTHX_ AV* av, bool abandon)
         /* abandon */
         const SSize_t size = AvFILLp(av) + 1;
         /* The ternary gives consistency with av_extend() */
-        AV *newav = newAV_alloc_x(size < PERL_ARRAY_NEW_MIN_KEY ?
+        AV *newav = newAV_alloc_xz(size < PERL_ARRAY_NEW_MIN_KEY ?
                                          PERL_ARRAY_NEW_MIN_KEY : size);
 #ifndef PERL_RC_STACK
         AvREIFY_only(newav);

t/op/goto.t

@@ -12,7 +12,7 @@ BEGIN {
 use warnings;
 use strict;
 use Config;
-plan tests => 133;
+plan tests => 134;
 our $TODO;
 
 my $deprecated = 0;
@@ -952,3 +952,23 @@ SKIP:
     my @a = bar_19936();
     is($XS::APItest::GIMME_V, 3, "GH #19936 gimme XS call");
 }
+
+# goto &sub could leave AvARRAY() slots of @_ uninitialised.
+
+{
+    my $i = 0;
+    my $f = sub {
+        goto &{ sub {} } unless $i++;
+        $_[1] = 1; # create a hole
+        # accessing $_[0] is more for valgrind/ASAN to chew on rather than
+        # we're too concerned about its value. Or it might give "bizarre
+        # copy" errors.
+        is($_[0], undef, "goto and AvARRAY");
+    };
+
+    # first call does goto, which gives &$f a fresh AV in pad[0],
+    # which formerly allocated an AvARRAY for it, but didn't zero it
+    $f->();
+    # second call creates hole in @_ which used to to be a wild SV pointer
+    $f->();
+}