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>

libavformat/rtpenc_h264.c

@@ -31,14 +31,14 @@
 
 static const uint8_t *avc_mp4_find_startcode(const uint8_t *start, const uint8_t *end, int nal_length_size)
 {
-    int res = 0;
+    unsigned int res = 0;
 
     if (end - start < nal_length_size)
         return NULL;
     while (nal_length_size--)
         res = (res << 8) | *start++;
 
-    if (start + res > end || res < 0 || start + res < start)
+    if (res > end - start)
         return NULL;
 
     return start + res;