mderef_u: fix MDEREF_INDEX_uoob run-time

we have a switch, which needs a static value.
uoob is currently only possible with const.
any :const padsv is converted to const before already.

this passes now all tests, but we still wrongly convert a
aelemfast_lex_u to mderef, the maybe_aelemfast logic is flawed.

Author: Reini Urban

op.c

@@ -14327,12 +14327,14 @@ S_maybe_multideref(pTHX_ OP *start, OP *orig_o, UV orig_action, U8 hints)
                                     DEBUG_kv(Perl_deb(aTHX_
                                         "mderef %s[%"IVdf"] shape ok -> uoob\n",
                                         PAD_COMPNAME_PV(targ), arg->iv));
+#if 0 /* already done in ck_aelem */
                                     if (arg->iv < 0) {
                                         arg->iv = AvFILLp(av)+1+arg->iv;
                                         DEBUG_kv(Perl_deb(aTHX_
                                             "mderef %s[->%"IVdf"]\n",
                                             PAD_COMPNAME_PV(targ), arg->iv));
                                     }
+#endif
                                 }
                                 index_type |= MDEREF_INDEX_uoob;
                             }

pp_hot.c

@@ -2418,6 +2418,7 @@ PP(pp_multideref)
                 switch (actions & MDEREF_INDEX_MASK) {
                 case MDEREF_INDEX_none:
                     goto finish;
+                case MDEREF_INDEX_const|MDEREF_INDEX_uoob:
                 case MDEREF_INDEX_const:
                     elem  = (++items)->iv;
                     break;