Prevent (negative) overflow of rm->remaining_len. This evaluation really only

Prevent (negative) overflow of rm->remaining_len. This evaluation really only
has two possible outcomes: either len and rm->remaining_len are the same, in
which case we care about the outcome and it is zero, or rm->remaining_len is
currently not in use and we don't care about the outcome. In that case, len
is positive and rm->remaining_len is zero, which leads to a negative result.
This is confusing and could eventually lead to a sign-flip if we skip a lot
of packets (unlikely, but still). Therefore, just always set it to zero.

Originally committed as revision 17919 to svn://svn.ffmpeg.org/ffmpeg/trunk