Improve errors in sceMpegRingbufferConstruct().

And also make Destruct() not actually null out the ringbuffer, per tests.
Fix mpeg struct header.
Correct errors in sceMpegRingbufferAvailableSize().
Fix warning in MIPS/MIPSAnalyst.cpp
Merge pull request #5029 from pal1000/work

Fix warning in MIPS/MIPSAnalyst.cpp
Merge pull request #5027 from unknownbrackets/mpeg-minor

Correct some basic mpeg functionality to match tests