Setting the IV directly with EVP_CipherInit_ex will
save two calls to openssl and simplify the code.

There is a comment that the 3 calls are required but
I am not sure why, EVP_CTRL_GCM_SET_IV_FIXED will
just store the vector and EVP_CTRL_GCM_IV_GEN will use
it internally and then increment the last digits before
returning it in iv variable passed in.
EVP_CipherInit_ex will store the iv and use it internally.
Incrementing and retrieving the new IV is not required for
SRTP-GCM, a new IV is used for each packet.

Tested with openssl 1.0.1d & 1.0.2g