qemu-char: minor mux chardev fixes

mux failure path has a memory leak.  creating a mux chardev can't
fail though, so just assert() that instead of fixing an error path
which never ever runs anyway ...

Also fix bid being leaked while being at it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
