lavf/utils: fix mem leak

Calling avformat_free_context() right after avformat_alloc_output_context2()
leaved option's default values not freed.
Options were freed only in av_write_trailer().

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>