gdbstub: Fix fd leak in gdbserver_open() error path

Fix a leak of a file descriptor in error exit paths in
gdbserver_open().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

gdbstub.c

@@ -2762,11 +2762,13 @@ static int gdbserver_open(int port)
     ret = bind(fd, (struct sockaddr *)&sockaddr, sizeof(sockaddr));
     if (ret < 0) {
         perror("bind");
+        close(fd);
         return -1;
     }
     ret = listen(fd, 0);
     if (ret < 0) {
         perror("listen");
+        close(fd);
         return -1;
     }
     return fd;