[core] Fix crash case in FEC filter

[sorayuki]

I met some crash when playing with this library.

FEC filter may access item out of range in rcv.cells.
FEC filter may access item out of range in rcv.rowq.

[sorayuki]

I don't know why appveyor CI report the branch as non-mergeable. Github says there is no conflicts.
It does fix the crash bugs; could I have my pull request reviewed?

[ethouris]

There's some problem with AppVeyor for now. Don't worry about it, your change isn't going to raise any portability issues.

However, there was a different fix for the crash in congestion control already provided; please upgrade to the latest master. The fix was also a little different.

[sorayuki]

done

[ethouris]

I'd have a question about line 1712, introduced new condition.

If this is going to generate an IPE then it's only a debug support, and this condition shouldn't have been satisfied in a non-buggy software (should be prevented from much earlier).

Have you found a case when you can see this log?

[sorayuki]

case:
[curl] - [srt tunnel] - [fujian, china] - [internet] - [tokyo, japan] - [srt tunnel] - [gost proxy]

srt socket runs in SRTT_LIVE mode which is not suitable for the http tunnel case, with FEC filter.

These are screenshot of crash report in Visual Studio 2019,
and the network traffic capture,
and the variable values when crash,
and the options of srt socket.

pr891-crash.zip