pp_index - create a SVt_PV directly, not via upgrade

Rather than create an `SVt_NULL`, then immediately to `sv_upgrade` it
(within the `sv_usepvn` call) to an SVt_PV, just create the SVt_PV in
the first place.

Author: richardleach

pp.c

@@ -3855,7 +3855,7 @@ PP(pp_index)
                  * the routine that contains the new byte string, and donate it
                  * to temp to ensure it will get free()d */
                 if (free_little_p) {
-                    little = temp = newSV_type(SVt_NULL);
+                    little = temp = newSV_type(SVt_PV);
                     sv_usepvn(temp, (char *) little_p, llen);
                     little_p = SvPVX_const(little);
                 }