It asserts that the frame linesize is larger than 37, but it can be
smaller and decoding such frames works.

Before commit cc884a35 src_stride > 7*MB_SIZE was necessary, because the
blocks were interleaved in the tmp buffer and the last block was added
with an offset of 6*MB_SIZE.
It was changed for src_stride <= 7*MB_SIZE to write the blocks
sequentially, hence the larger tmp_step.
After that the assert was only necessary to make sure that the buffer
remained large enough.
Since commit bd2b6b33 s->scratchbuf is used as tmp buffer.
As part of commit 86e107a7 the minimal scratchbuf size was increased to
256*7*MB_SIZE, which is enough for any src_stride <= 7*MB_SIZE.

Also add a comment explaining the tmp_step calculation.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>