[perl #119949] Stop undef *_, goto &sub from crashing

Commit 049bd5f fixed problems with the wrong @_ being visible
after *_ modification followed by goto.  In so doing, it made it
possible for a null to be placed at the start of the target sub’s
pad, because it was not checking that the array it got from PL_defgv
was actually non-null.  Simply adding the check makes everything work.

Conflicts:
	t/op/goto.t

pp_ctl.c

@@ -2956,8 +2956,10 @@ PP(pp_goto)
 		       to freed memory as the result of undef *_.  So put
 		       it in the callee’s pad, donating our refer-
 		       ence count. */
-		    SvREFCNT_dec(PAD_SVl(0));
-		    PAD_SVl(0) = (SV *)(cx->blk_sub.argarray = arg);
+		    if (arg) {
+			SvREFCNT_dec(PAD_SVl(0));
+			PAD_SVl(0) = (SV *)(cx->blk_sub.argarray = arg);
+		    }
 
 		    /* GvAV(PL_defgv) might have been modified on scope
 		       exit, so restore it. */

t/op/goto.t

@@ -10,7 +10,7 @@ BEGIN {
 
 use warnings;
 use strict;
-plan tests => 89;
+plan tests => 91;
 our $TODO;
 
 my $deprecated = 0;
@@ -482,6 +482,23 @@ sub { *__ = \@_;  goto &null } -> ("rough and tubbery");
 is ${*__}[0], 'rough and tubbery', 'goto &foo leaves reified @_ alone';
 
 
+# goto &perlsub when @_ itself does not exist [perl #119949]
+# This was only crashing when the replaced sub call had an argument list.
+# (I.e., &{ sub { goto ... } } did not crash.)
+sub {
+    undef *_;
+    goto sub {
+	is *_{ARRAY}, undef, 'goto &perlsub when @_ does not exist';
+    }
+}->();
+sub {
+    local *_;
+    goto sub {
+	is *_{ARRAY}, undef, 'goto &sub when @_ does not exist (local *_)';
+    }
+}->();
+
+
 # [perl #36521] goto &foo in warn handler could defeat recursion avoider
 
 {