Add some comments and tweak assertions for cbc.c.

See https://boringssl-review.googlesource.com/#/c/4832/.

Change-Id: Icf457a2b47bc2d5b84dddc454d5ca8ec328b5169
Reviewed-on: https://boringssl-review.googlesource.com/4860
Reviewed-by: Adam Langley <agl@google.com>
1 file changed