[keepalive] Fix assert when closing module immediately after startup.
[keepalive] Fix assert when closing module immediately after startup.

If the keepalive starts pending call and is immediately unloaded the
pending call is not cancelled and the pending call function is called
after the pulseaudio dbus connection object has ref count 0, causing
assert fail.

mer-hybris/pulseaudio-modules-droid,jusa/pulseaudio-modules-droid,mer-hybris/pulseaudio-modules-droid,jusa/pulseaudio-modules-droid