libavformat/mov.c: use calculated dts offset when seeking in streams

Subtract the calculated dts offset from the requested timestamp before
seeking. This fixes an error "Error while filtering: Operation not
permitted" observed with a short file which contains only one key frame
and starts with negative timestamps.

Then, av_index_search_timestamp() returns a valid negative timestamp,
but mov_seek_stream bails out with AVERROR_INVALIDDATA.

Fixes ticket #6139.

Signed-off-by: Jonas Licht <jonas.licht@fem.tu-ilmenau.de>
Signed-off-by: Peter Große <pegro@friiks.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
