In AudioPulseAudio::streamWriteCallback() we're operating on pcmbuf
via an operation from CPU class. All methods of CPU assume the buffer
being aligned on 16 byte boundaries. However pa_xmalloc() & friends do
not allocate aligned memory which resulted in a crash when calling
CPU::convertToS16() (SSE2 version takes advantage of aligned memory).
Replacing pa_xmalloc()/pa_xfree() with CPU::memAlloc()/CPU::memFree()
fixes this crash.
Closes bug #2890465.