IMPL Some Mpeg

ref https://github.com/hrydgard/ppsspp/issues/508
please help to check whether I IMPL correctly
Add a comment that it's just a random address
Add a debug log for sceMpegAvcResourceFinish
Handle  ringbufferAddr =0 in sceMpegCreate
Remove PSP_ERROR_MPEG_INVALID_VALUE in scempeg.cpp
Merge pull request #867 from sum2012/master

IMPL Some Mpeg and Handle ringbufferAddr =0 in sceMpegCreate