should always initialize header flags for pooled strings

src/state.c

@@ -174,6 +174,7 @@ mrb_str_pool(mrb_state *mrb, mrb_value str)
     ns->as.heap.aux.capa = 0;
   }
   else {
+    ns->flags = 0;
     if (s->flags & MRB_STR_EMBED) {
       ptr = s->as.ary;
       len = (mrb_int)((s->flags & MRB_STR_EMBED_LEN_MASK) >> MRB_STR_EMBED_LEN_SHIFT);
@@ -193,7 +194,6 @@ mrb_str_pool(mrb_state *mrb, mrb_value str)
       ns->as.ary[len] = '\0';
     }
     else {
-      ns->flags = 0;
       ns->as.heap.ptr = (char *)mrb_malloc(mrb, (size_t)len+1);
       ns->as.heap.len = len;
       ns->as.heap.aux.capa = len;