check return value of `write` to suppress warning.

* ext/-test-/gvl/call_without_gvl/call_without_gvl.c (do_loop):
  `-Wunused-result` compiler option (gcc?) checks checking
  return value of `write`.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67063 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/-test-/gvl/call_without_gvl/call_without_gvl.c

@@ -38,7 +38,8 @@ do_loop(void *p)
     struct loop_ctl *ctl = p;
 
     /* tell the waiting process they can interrupt us, now */
-    write(ctl->notify_fd, "", 1);
+    int err = write(ctl->notify_fd, "", 1);
+    if (err == -1) rb_bug("write error");
 
     while (!ctl->stop) {
         struct timeval tv = { 0, 10000 };