#5 SRTCP index should start at 0, not 1

[Per Cederqvist]

RFC 3711 says (3.4. Secure RTCP):

The SRTCP index MUST be set to zero before the
first SRTCP packet is sent, and MUST be incremented
by one, modulo 2^31, after each SRTCP packet is
sent.

libsrtp increments the index before it sends the
packet, resulting in the first SRTCP packet having
an index of 1, not 0. The untested enclosed patch
should fix this.

[Per Cederqvist]

Update SRTCP index per RFC 3711.