spicevmc: Drop unsent data on client disconnection

When redirecting a USB webcam over a slow link, it's currently possible
to hit an assertion in spice-server by running cheese (application using
the webcam), killing the client with ctrl+c and then restarting the
client:
qemu-kvm: spicevmc.c:324: spicevmc_red_channel_alloc_msg_rcv_buf:
Assertion `!state->recv_from_client_buf' failed.

This happens when red_peer_handle_incoming tries to allocate memory for
a message using spicevmc:
handler->msg = handler->cb->alloc_msg_buf(handler->opaque, msg_type,
msg_size);

red_peer_handle_incoming() is called when there is client data to be
read, and does
- call alloc_msg_buf() to allocate memory for the message
- read the message
- if the read was partial, return early, the main loop will call again
  red_peer_handle_incoming() when there is more data available for that
  channel
- parse the message
- call release_msg_buf() to free the message

For channels based on spicevmc (usbredir and port), alloc_msg_buf()
stores message data in SpiceVmcState::recv_from_client_buf and before
allocating new memory, it asserts that it's NULL.

This is what causes this crash in the following scenario:
- SpiceVmc::alloc_msg_buf() is called and allocates memory for a new
  message in SpiceVmcState::recv_from_client_buf
- red_peer_handle_incoming() returns early as all the spicevmc message
  data hasn't been received yet
- the client gets killed
- the main channel notices the disconnect and calls
  main_dispatcher_client_disconnect() which will disconnect all the
  channels
- SpiceVmc::on_disconnect is called
- after the new client connects, SpiceVmc::alloc_msg_buf() is called,
  notices that SpiceVmcState::recv_from_client_buf is already set, and
  asserts()

This commit makes sure the partial SpiceVmcState::recv_from_client_buf
data is cleared on disconnect so that the assert does not trigger.

This fixes https://bugzilla.redhat.com/show_bug.cgi?id=1264113

server/spicevmc.c

@@ -223,6 +223,11 @@ static void spicevmc_red_channel_client_on_disconnect(RedChannelClient *rcc)
     sin = state->chardev_sin;
     sif = SPICE_CONTAINEROF(sin->base.sif, SpiceCharDeviceInterface, base);
 
+    if (state->recv_from_client_buf) { /* partial message which wasn't pushed to device */
+        spice_char_device_write_buffer_release(state->chardev_st, state->recv_from_client_buf);
+        state->recv_from_client_buf = NULL;
+    }
+
     if (state->chardev_st) {
         if (spice_char_device_client_exists(state->chardev_st, rcc->client)) {
             spice_char_device_client_remove(state->chardev_st, rcc->client);