tests: vhost-user-test: initialize 'fd' in chr_read

Currently when processing VHOST_USER_SET_VRING_CALL
if 'qemu_chr_fe_get_msgfds' get no fd, the 'fd' will
be a stack uninitialized value.
Signed-off-by: NLi Qiang <liq3ea@163.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

tests/vhost-user-test.c

@@ -309,7 +309,7 @@ static void chr_read(void *opaque, const uint8_t *buf, int size)
     CharBackend *chr = &s->chr;
     VhostUserMsg msg;
     uint8_t *p = (uint8_t *) &msg;
-    int fd;
+    int fd = -1;
 
     if (s->test_fail) {
         qemu_chr_fe_disconnect(chr);