Perl_class_wrap_method_body: simple push to newly-created AV

Since the AV has just been created and is simple in nature, there's
no need to do a full-blown `av_push`.

Technically, that's still overkill. Simplification could be taken
further by creating the `fieldmap` AV with `PadnamelistMAX(pnl) - 2`
items (when that is a >0 value), then directly assigning values into
elements. But it's not clear (to me) if that would be worth doing.

class.c

@@ -857,8 +857,8 @@ Perl_class_wrap_method_body(pTHX_ OP *o)
         if(fieldix > max_fieldix)
             max_fieldix = fieldix;
 
-        av_push(fieldmap, newSVuv(padix));
-        av_push(fieldmap, newSVuv(fieldix));
+        av_push_simple(fieldmap, newSVuv(padix));
+        av_push_simple(fieldmap, newSVuv(fieldix));
     }
 
     UNOP_AUX_item *aux = NULL;