alsa_sound: Fix handling of zero size write request

Previously, periodSize number of bytes from the buffer pointer
location were always written to PCM even in the case when the
write call specified 0 bytes buffer size.
This caused short pop noise at the beginning of the playback
after the "audio flinger: force audio path start from normal mixer"
fake write() of 0 bytes introduction in audioflinger.

Change-Id: Ifeb353fed01a2f2b7e771a7ccb7d422f77a69228