sync fstring_table for deletion

Ractors can access this table simultaneously so we need to sync
accesses.

string.c

@@ -1378,8 +1378,13 @@ rb_str_free(VALUE str)
 {
     if (FL_TEST(str, RSTRING_FSTR)) {
 	st_data_t fstr = (st_data_t)str;
-	st_delete(rb_vm_fstring_table(), &fstr, NULL);
-	RB_DEBUG_COUNTER_INC(obj_str_fstr);
+
+        RB_VM_LOCK_ENTER();
+        {
+            st_delete(rb_vm_fstring_table(), &fstr, NULL);
+            RB_DEBUG_COUNTER_INC(obj_str_fstr);
+        }
+        RB_VM_LOCK_LEAVE();
     }
 
     if (STR_EMBED_P(str)) {