listener: unlock lev on error in listener_read_cb()

Without this patch:
  $ regress --no-fork +listener/error_unlock
  listener/error_unlock: [warn] Error from accept() call: Too many open files
  [err] ../evthread.c:220: Assertion lock->count == 0 failed in ../evthread.c
  Aborted (core dumped)

Fixes: libevent#341
Fixes: listener/error_unlock

listener.c

@@ -442,6 +442,7 @@ listener_read_cb(evutil_socket_t fd, short what, void *p)
 		listener_decref_and_unlock(lev);
 	} else {
 		event_sock_warn(fd, "Error from accept() call");
+		UNLOCK(lev);
 	}
 }