In the defined AEAD modes, SRTP packets must always be encrypted and
authenticated, but SRTCP packets may be only authenticated.  It's
possible, therefore, for us to end up in `srtp_protect_aead()` without
the `sec_serv_conf` bit being set.  We should just ignore this and
encrypt the RTP packet anyway.

What we are doing instead is encrypting the packet anyway, but setting
`enc_start` to NULL first.  This causes `aad_len` to underflow which
will cause us to over-read in `cipher_set_aad()`.

If we could get past that, we would try to read and write memory
starting at 0x0 down in `cipher_encrypt()`.

This commit causes us to not check the `sec_serv_conf` bit and never
set `enc_start` to NULL in `srtp_protect_aead()`.

`srtp_unprotect_aead()` does not contain a similar error.