core: Work around -Wlogical-not-parentheses warnings

core: Work around -Wlogical-not-parentheses warnings

pulsecore/sink.c: In function 'pa_sink_put':
pulsecore/sink.c:648:53: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
     pa_assert(!(s->flags & PA_SINK_DYNAMIC_LATENCY) == (s->thread_info.fixed_latency != 0));
                                                     ^

pulsecore/source.c: In function 'pa_source_put':
pulsecore/source.c:599:55: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
     pa_assert(!(s->flags & PA_SOURCE_DYNAMIC_LATENCY) == (s->thread_info.fixed_latency != 0));
                                                       ^
rewrite expression to suppress warning:
!(x & MASK) == (y != 0)
<->
!(x & MASK) == !(y == 0)

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