Fix misspelling in comment
Prevent buffer overflow from untrusted RTP/SRTP lengths

When computing the start address of the RTP data to encrypt or SRTP
data to decrypt (`enc_start`), we are using `hdr->cc` (the CSRC
count), which is untrusted data from the packet, and the length field
of an RTP header extension, which is also untrusted and unchecked data
from the packet.

This value then pollutes our calculation of how much data we'll be
encrypting or decrypting (`enc_octet_len`), possibly causing us to
underflow.

We'll then call `cipher_encrypt()` or `cipher_decrypt()` with these
two values, causing us to read from and write to arbitrary addresses
in memory.

(In the AEAD functions, we'd also pollute `aad_len`, which would cause
us to read undefined memory in `cipher_set_aad`.)

This commit adds checks to verify that the `enc_start` we calculate is
sane based on the actual packet length.
Avoid buffer over-read on null cipher AEAD

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.
Avoid buffer-overflow on short RTCP/SRTCP packets

In `srtp_unprotect_rtcp()` we are not validating that the packet
length is as long as the minimum required.  This would cause
`enc_octet_len` to underflow, which would cause us to try to decrypt
data past the end of the packet in memory -- a buffer over-read and
buffer overflow.

In `srtp_protect_rtcp()`, we were similarly not validating the packet
length.  Here we were also polluting the address of the SRTCP
encrypted flag and index (the `trailer`), causing us to write one word
to a bogus memory address before getting to the encryption where we
would also overflow.

In this commit we add checks to appropriately validate the RTCP/SRTCP
packet lengths.

`srtp_unprotect_rtcp_aead()` (but not protect) did correctly validate
the packet length; this check would now be redundant as the check in
`srtcp_unprotect_rtcp()` will also run first, so it has been removed.
Merge pull request #53 from traviscross/master

Reviewed the code and it looks good.  Thanks for the contribution.