channel-usbredir: Fix crash on channel-up

By adding a guard to not handle channel-up on SpiceUsbredirChannel in
case struct usbredirhost wasn't initialized yet. Same guard is in
place for the generic usbredir_handle_msg() function to avoid handling
Server's message while Client's initialization is not done.

As mentioned in commit 291f3e44, this isn't a problem for
graphical clients as some initialization is done to present the
shareable usb devices to user.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1399838


Signed-off-by: Victor Toso <victortoso@redhat.com>
Acked-by: Christophe Fergeau <cfergeau@redhat.com>

src/channel-usbredir.c

@@ -817,6 +817,7 @@ static void spice_usbredir_channel_up(SpiceChannel *c)
     SpiceUsbredirChannel *channel = SPICE_USBREDIR_CHANNEL(c);
     SpiceUsbredirChannelPrivate *priv = channel->priv;
 
+    g_return_if_fail(priv->host != NULL);
     /* Flush any pending writes */
     usbredirhost_write_guest_data(priv->host);
 }