Fixing memory and descriptor leakage in case of port send failure.

In rare cases, when the destination process had finished running but no
notification of this was received yet, send could fail with an error, and the
send message structure with file descriptors could leak.

The leakage was periodically reproduced by respawn tests on FreeBSD 12.

src/nxt_port_socket.c

@@ -524,6 +524,24 @@ next_fragment:
 
         } else {
             if (nxt_slow_path(n == NXT_ERROR)) {
+                if (msg->link.next == NULL) {
+                    if (msg->close_fd) {
+                        if (msg->fd[0] != -1) {
+                            nxt_fd_close(msg->fd[0]);
+
+                            msg->fd[0] = -1;
+                        }
+
+                        if (msg->fd[1] != -1) {
+                            nxt_fd_close(msg->fd[1]);
+
+                            msg->fd[1] = -1;
+                        }
+                    }
+
+                    nxt_port_release_send_msg(msg);
+                }
+
                 goto fail;
             }