rtpenc: fix overflow checking in avc_mp4_find_startcode()

The check `start + res < start' is broken since pointer overflow is
undefined behavior in C.  Many compilers such as gcc/clang optimize
away this check.

Use `res > end - start' instead.  Also change `res' to unsigned int
to avoid signed left-shift overflow.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
