Fix cast to include entire expression
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.

yarrcc/libsrtp-ios,ksophocleous/libsrtp,cuijinquan/libsrtp,yarrcc/libsrtp-ios,cuijinquan/libsrtp,persmule/libsrtp,cuijinquan/libsrtp,yarrcc/libsrtp-ios,ksophocleous/libsrtp,persmule/libsrtp,persmule/libsrtp