Fix calculation of RTP/SRTP packet end address

Our validation of `enc_start` was subtly wrong; we were adding a length
in octets as a length in 4 byte words.  This would allow underflow of
`enc_octet_len`.  This commit corrects the calculation.

Relates-to: fe367616718862a228f5859b83c383593dd4b79d
Fix cast to include entire expression

Elsewhere in the code this calculation casts the entire result to an
unsigned int.  Here we were just casting the octet length of the packet
before doing the subtraction, which has different semantics.  Casting
the entire expression seems more reasonable.
Simplify calculation of encrypted octet length

We were calculating the `enc_octet_len` value by computing an offset
in 4-byte words, then left shifting by 2 so we could subtract from a
length in octets.  This was a bit opaque.  We'll let the compiler do
this work instead by computing the offset in octets.
Add missing `char` to declaration

The bit-fields of these structs are defined as unsigned chars
apparently to appease Microsoft's compiler.  One field in the struct
for big-endian systems was missing this declaration.
Say `unsigned int` rather than `unsigned`

`unsigned int` == `unsigned` in C, but we were using both forms.  This
commit standardizes us to using `unsigned int`.
Validate passed in signed length value

In other functions we check right away that the `pkt_octet_len` is
greater than zero, but in `srtp_unprotect_rtcp` we were waiting until
we knew the tag length, at which point we had already used the value.
We'll do an initial check here to make sure the value is sane.
Use `unsigned int` internally for `pkt_octet_len`

Now that we always validate that the passed in length is positive, we
can treat the value as unsigned deeper in our code.
Avoid compiler warning about uninitialized value

The `verbose` value here will never be used uninitialized because the
branches in which it would not be set lead us to call exit().  But
gcc-4.8 currently can't see that.
Merge pull request #55 from traviscross/master

Thanks for the contribution.  I am seeing one compiler warning with this commit.  I'll fix it after the merge.