Teach mro code about null array elements

This is part of ticket #119433.

Commit ce0d49f changed AVs to use NULL for nonexistent elements.  The
mro lookup code was not accounting for that, causing Class::Contract’s
tests to crash (and perhaps other modules, too).

1 files changed, 2 insertions, 1 deletions

diff --git a/mro.c b/mro.c
index 2ce9fa2da3..18dfa8c011 100644
--- a/mro.c
+++ b/mro.c
@@ -269,10 +269,11 @@ S_mro_get_linear_isa_dfs(pTHX_ HV *stash, U32 level)
 
         /* foreach(@ISA) */
         while (items--) {
-            SV* const sv = *svp++;
+            SV* const sv = *svp ? *svp : &PL_sv_undef;
             HV* const basestash = gv_stashsv(sv, 0);
 	    SV *const *subrv_p;
 	    I32 subrv_items;
+	    svp++;
 
             if (!basestash) {
                 /* if no stash exists for this @ISA member,