lavf: compute probe buffer size more reliably.

lavf: compute probe buffer size more reliably.

The previous code computes the offset by reversing the growth
of the allocated buffer size: it is complex and did lead to
inconsistencies when the size limit is reached.

Fix trac ticket #1991.
(cherry picked from commit 03847eb8259291b4ff1bd840bd779d0699d71f96)

Conflicts:
	libavformat/utils.c
(cherry picked from commit e74cd2f4706f71da5e9205003c1d8263b54ed3fb)