mux: fix ctrl-a b again

Commit fb5e19d2 originally fixed the
regression, but was inadvertently broken again in merge commit
2d6752d3.

Fixes:
https://bugs.launchpad.net/qemu/+bug/1654137



Cc: qemu-stable@nongnu.org
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180515152500.19460-3-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>