Fix decoding ATI VCR2 codec, sample in
samples/V-codecs/ATI-VCR12/ATI/VCR2test.avi
Software decoding was broken since r8865, when two new context variables
were added, but not initialized in vcr2_init_sequence().
Another such context variable was added in r18381.

Originally committed as revision 21438 to svn://svn.ffmpeg.org/ffmpeg/trunk
