Bug 1075640 - Don't return 0-length frames for decoding; add comments about loss handling. r=ehugg, a=sledru

Fixes OpenH264 crashes, especially under extreme packet loss

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branch : GECKO330_2014101104_RELBRANCH
Bug 1075640 - Check for zero length frames in GMP H264 decode. r=jesup, a=sledru

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branch : GECKO330_2014101104_RELBRANCH