FIX(client): Fix PulseAudio regression by nulling buffer

In b5a67c0 a buffer in the PulseAudio was made static, but
at the same time the memset(0) was removed.
In combination, that lead to a bug where the last few frames
of any given audio played back was repeated until something
else filled the buffer.

This commit adds a simple fill to the buffer where the memset(0)
previously was fixing the bug.

src/mumble/PulseAudio.cpp

@@ -660,6 +660,8 @@ void PulseAudioSystem::write_callback(pa_stream *s, size_t bytes, void *userdata
 		pas->bAttenuating = (Global::get().bAttenuateOthers || Global::get().s.bAttenuateOthers);
 
 	} else {
+		std::fill(buffer.begin(), buffer.end(), 0);
+
 		// attenuate if intructed to (self-activated)
 		pas->bAttenuating = Global::get().bAttenuateOthers;
 	}