core: Annotate variables only used within assert()s to be PA_UNUSED

core: Annotate variables only used within assert()s to be PA_UNUSED

supresses a warning when compiling with NDEBUG:

pulsecore/aupdate.c: In function 'pa_aupdate_read_end':
pulsecore/aupdate.c:82:14: warning: variable 'n' set but not used [-Wunused-but-set-variable]
     unsigned n;

pulsecore/sink-input.c: In function 'pa_sink_input_unlink':
pulsecore/sink-input.c:648:27: warning: variable 'p' set but not used [-Wunused-but-set-variable]
     pa_source_output *o, *p = NULL;

pulsecore/sink-input.c: In function 'find_filter_sink_input':
pulsecore/sink-input.c:1523:14: warning: unused variable 'i' [-Wunused-variable]
     unsigned i = 0;

pulsecore/sink-input.c: In function 'pa_sink_input_start_move':
pulsecore/sink-input.c:1569:27: warning: variable 'p' set but not used [-Wunused-but-set-variable]
     pa_source_output *o, *p = NULL;

  CC       pulsecore/libpulsecore_5.0_la-sink.lo
pulsecore/sink.c: In function 'pa_sink_unlink':
pulsecore/sink.c:673:24: warning: variable 'j' set but not used [-Wunused-but-set-variable]
     pa_sink_input *i, *j = NULL;

   pulsecore/source-output.c: In function 'find_filter_source_output':
pulsecore/source-output.c:1179:9: warning: unused variable 'i' [-Wunused-variable]
     int i = 0;

  CC       pulsecore/libpulsecore_5.0_la-source.lo
pulsecore/source.c: In function 'pa_source_unlink':
pulsecore/source.c:616:27: warning: variable 'j' set but not used [-Wunused-but-set-variable]
     pa_source_output *o, *j = NULL;

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>