avcodec/cfhd: Increase minimum band dimension to 3

The implementation does not currently support len=2

Fixes out of array accesses
Fixes: 29d1b3db5ba2205e82b0b3a533e057a3/asan_heap-oob_12b650c_9254_3b8c4e4d931eb2c32841c18ebb297f1d.avi

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
