Add cast, to fix GCC warning if 'size_t' is not 'long'.
Add cast, to fix GCC warning if 'size_t' is not 'long'.

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