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)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>

Conflicts:
	libavformat/utils.c

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit 8575f5362f98c937758b20ff8512d6767a56208e)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>
Prepare for 0.8.11 Release