Fix a memory leak for WBuffers used in channel_write().

channel_write() uses a ref-counted buffer for writing. This buffer
should be released if it was used in "refcount" channel_write() calls.
But calling channel_write() on a closed channel would return early and
not decrease the refcount of the used buffer.

src/nvim/msgpack_rpc/channel.c

@@ -495,6 +495,7 @@ static bool channel_write(Channel *channel, WBuffer *buffer)
   bool success;
 
   if (channel->closed) {
+    wstream_release_wbuffer(buffer);
     return false;
   }