lavf: simplify handling of offset in av_probe_input_buffer()

(cherry picked from commit c1868e7ee7b07b40a0fe15f50df89fe499a01a50)

Signed-off-by: Anton Khirnov <anton@khirnov.net>
lavf: use a fixed width type

It's shorter and more consistent with the rest of the code.

(cherry picked from commit 8b76362836f3c373c3aadc544522edcbef16dd5f)

Signed-off-by: Anton Khirnov <anton@khirnov.net>
lavf: make av_probe_input_buffer more robust

Always use the actually read size as the offset instead of making
possibly invalid assumptions.

Addresses: CVE-2012-6618

(cherry picked from commit 2115a3597457231a6e5c0527fe0ff8550f64b733)

Conflicts:
	libavformat/utils.c

Signed-off-by: Anton Khirnov <anton@khirnov.net>