intreadwrite.h: fix AV_RL32/AV_RB32 signedness.

The output type of the AV_RL32/AV_RB32 macros was signed int. The
resulting overflow broke at least some ASF streams with large
timestamps. Fix by adding a cast to uint32_t.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>