Initialize RStruct fields obj to 0

Author: byroot

struct.c

@@ -823,6 +823,9 @@ struct_alloc(VALUE klass)
         if (RCLASS_MAX_IV_COUNT(klass) == 0 && embedded_size == rb_gc_obj_slot_size((VALUE)st)) {
             FL_SET_RAW((VALUE)st, RSTRUCT_FL_GENIVAR);
         }
+        else {
+            RSTRUCT_SET_FIELDS_OBJ((VALUE)st, 0);
+        }
 
         rb_mem_clear((VALUE *)st->as.ary, n);