avformat/mov: only force parsing for video tracks if stss is empty

Fixes playback of some AAC streams, which are otherwise mangled by the
parser, and stss is typically only valid for video anyway.

Fixes a regression since e41ea866.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 019247bdc326a90bf20d3ce5d2413cc642e8bb08)
avformat/mov: fix keyframe flags for sample from chromium Issue 340865

Fixes ticket #3362.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit a0911b059763b8f13c70adcbbe71e10382855104)