Fix srtp_t initialization. In order to let master key to be different for the RTP and the RTCP streams (which will happen with SRTP-DTLS and no rtcp-mux), we use one srtp_t for RTP and one srtp_t for RTCP. However, the derivation of keys from master key requires both SRTP and SRTCP parameters to be known, which was not the case. Indeed, the code was only initializing the SRTCP parameters for the srtp_t responsible for RTCP. This leads to a wrong computation of the SRTCP auth key, which then creates systematic srtp_unprotect_rtcp() errors.