qemu-char: avoid potential double-free

If tcp_set_msgfds() is called several time with NULL fds, this
could lead to double-free.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

qemu-char.c

@@ -2697,6 +2697,7 @@ static int tcp_set_msgfds(CharDriverState *chr, int *fds, int num)
     }
     /* clear old pending fd array */
     g_free(s->write_msgfds);
+    s->write_msgfds = NULL;
 
     if (num) {
         s->write_msgfds = g_new(int, num);