Validate passed in signed length value
Validate passed in signed length value

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.

cuijinquan/libsrtp,persmule/libsrtp,yarrcc/libsrtp-ios,cuijinquan/libsrtp,yarrcc/libsrtp-ios,ksophocleous/libsrtp,ksophocleous/libsrtp,persmule/libsrtp,persmule/libsrtp,cuijinquan/libsrtp,yarrcc/libsrtp-ios