H.261 decoder improvements

- GOB's can exist of only MBA_STUFFING codes: it crashed before, but it is fixed now
- Clearer code
- Some extra checks so the decoder is more resilient against errors
patch by (Maarten Daniels <maarten dot daniels at student dot luc dot ac dot be>)

Originally committed as revision 3457 to svn://svn.ffmpeg.org/ffmpeg/trunk