Fix Theora decoding bug if loopfilter is completely disabled
Bug caused by the fact that get_bits(gb, 0) is undefined.
Doesn't affect any streams generated by the official Theora encoder, but such
streams are nevertheless valid.

Fixes decoding of CELT-933dd833-nmr-bandt.ogv.

Originally committed as revision 25573 to svn://svn.ffmpeg.org/ffmpeg/trunk
