Fix pa_rtclock_from_wallclock

Fix pa_rtclock_from_wallclock

The HAVE_CLOCK_GETTIME macro protects timespec and related functions, nothing of which is used in
pa_rtclock_from_wallclock.  And silently just not converting was not the proper solution anyway.

Also add an assert in pulse/mainloop.c to report the integer overflow that was triggered by the wrong
pa_rtclock_from_wallclock.  Without the assert, debugging was painful.