snow: remove an obsolete av_assert2

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>
(cherry picked from commit 3526a120f9)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/snow.h

@@ -296,6 +296,8 @@ static av_always_inline void add_yblock(SnowContext *s, int sliced, slice_buffer
     BlockNode *lb= lt+b_stride;
     BlockNode *rb= lb+1;
     uint8_t *block[4];
+    // When src_stride is large enough, it is possible to interleave the blocks.
+    // Otherwise the blocks are written sequentially in the tmp buffer.
     int tmp_step= src_stride >= 7*MB_SIZE ? MB_SIZE : MB_SIZE*src_stride;
     uint8_t *tmp = s->scratchbuf;
     uint8_t *ptmp;
@@ -339,8 +341,6 @@ static av_always_inline void add_yblock(SnowContext *s, int sliced, slice_buffer
 
     if(b_w<=0 || b_h<=0) return;
 
-    av_assert2(src_stride > 2*MB_SIZE + 5);
-
     if(!sliced && offset_dst)
         dst += src_x + src_y*dst_stride;
     dst8+= src_x + src_y*src_stride;