Eat cycles in sceMpegRingbufferAvailableSize().

Some games call this in a loop, so it affects perf.  And, this eats quite
a few cycles, implying it does a lot more work than I thought it did.
Clean up sceMpegRingbufferAvailableSize().

Avoid a memory copy, some crash checks.
Merge pull request #2401 from unknownbrackets/mpeg-minor

Eat cycles in sceMpegRingbufferAvailableSize() and cut down memcpy()