Make vivify_defelem allow &PL_sv_undef array entries

This is something I failed to change in commit ce0d59f.  I don’t know
of a way to trigger this in pure-Perl code, hence the use of XS in
the test.  It did show up in pure-Perl code due to a bug fixed by the
previous commit.

1 files changed, 1 insertions, 1 deletions

diff --git a/mg.c b/mg.c
index 604fe6bb05..543af8152a 100644
--- a/mg.c
+++ b/mg.c
@@ -2381,7 +2381,7 @@ Perl_vivify_defelem(pTHX_ SV *sv)
 	    LvTARG(sv) = NULL;	/* array can't be extended */
 	else {
 	    SV* const * const svp = av_fetch(av, LvSTARGOFF(sv), TRUE);
-	    if (!svp || (value = *svp) == &PL_sv_undef)
+	    if (!svp || !(value = *svp))
 		Perl_croak(aTHX_ PL_no_aelem, LvSTARGOFF(sv));
 	}
     }