srtp: fix for possible sequence number corruption on wrap around boun…

[nifigase]

We encountered and fixed a bug in libre:
Assume we protect with SRTP a sequence of packets with following sequence numbers:
..., 65533, 65535, 0, 1, 65534, 2, ...
Sequence is a little bit out of order, and roc (roll over counter) should increment by 1.
The issue was that it eventually incremented by 2:
SN 0: roc++; s_l = 0
SN 1: s_l = 1
SN 65534: (hdr.seq > strm->s_l) => s_l = 65534
SN 2: roc++; s_l = 2

That led to incorrect "ix" calculation,then incorrect authentication tag was formed, then receiver (i.e. web browser based on libSRTP) failed to unprotect all subsequent packets with err=7 (auth error), stream was frozen.

[alfredh]

hi @nifigase

I am the co-maintainer of the libre project, and I just stumbled upon this PR
by chance.

The official libre project is maintained here:

https://github.com/creytiv/re/pulls

Could you please move this PR to that project. You have found a real bug
so it would be good to fix it.

@traviscross perhaps you should consider deleting this project as it is just
confusing people. If you want to keep the project, perhaps it is better to
clone it from creytiv/re.

Alfred