avformat/aviobuf: Don't check for overflow after it happened

If adding two ints overflows, it doesn't matter whether the result will
be stored in an unsigned or not; and checking afterwards does not make it
retroactively defined.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 28a078eded1c29985ed078b59d48ff59cf00394b)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/aviobuf.c

@@ -1275,7 +1275,7 @@ static int dyn_buf_write(void *opaque, uint8_t *buf, int buf_size)
     unsigned new_size, new_allocated_size;
 
     /* reallocate buffer if needed */
-    new_size = d->pos + buf_size;
+    new_size = (unsigned)d->pos + buf_size;
     new_allocated_size = d->allocated_size;
     if (new_size < d->pos || new_size > INT_MAX/2)
         return -1;