Validate passed in signed length value (again)

In other functions we check right away that the `pkt_octet_len` is
greater than zero, but in `srtp_unprotect_rtcp` we were waiting until
we knew the tag length, at which point we had already used the value.
We'll do an initial check here to make sure the value is sane.

We tried adding this check back in commit
444a544c0c525c1fe6ccc560ae2ddcf38e6a461c, but that commit erroneously
used the tag length in the calculation.  The comment above it made it
clear that was unintended.

This check must come before we compute the tag length or we'll have
cast the `*pkt_octet_length` to an unsigned value in the call to
`srtp_stream_init_from_ekt()` without first checking that the value is
greater than zero.

This reverts the non-whitespace changes in commit
73e3093fcc309328aeb1201373e30ade6137b9f2.
Merge pull request #57 from traviscross/master

Thanks for the contribution.