sink/source: Don't update default sink/source before calling PA_CORE_HOOK_{SINK,SOURC...

sink/source: Don't update default sink/source before calling PA_CORE_HOOK_{SINK,SOURCE}_PUT

In sink_put() and source_put(), pa_core_update_default_{sink,source}() was called
before the PA_CORE_HOOK_{SINK,SOURCE}_PUT hook. Therefore module-switch-on-connect
could not correctly determine the old default sink/source if no user default was
set and a sink/source with higher priority than any other sink/source turned up.

This patch corrects the problem by swapping the order of the hook call and the
pa_core_update_default_sink() call.

Additionally it corrects a problem in module-switch-on-connect. If, after the
change above, the new sink/source was the first sink/source to appear, pulseaudio
would crash because module-switch-on-connect assumed that the default sink/source
was not NULL. The patch checks if the default sink/source is NULL and only sets
the new default sink/source in that case.