Fix calculation of RTP/SRTP packet end address
Fix calculation of RTP/SRTP packet end address

Our validation of `enc_start` was subtly wrong; we were adding a length
in octets as a length in 4 byte words.  This would allow underflow of
`enc_octet_len`.  This commit corrects the calculation.

Relates-to: fe367616718862a228f5859b83c383593dd4b79d

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