libavformat/mov: Elimitate double reading of COVR metadata if MOV_EXPORT_ALL_METADATA is enabled

Problem: ffmpeg tries to read COVR atom data twice if MOV_EXPORT_ALL_METADATA is enabled.
If COVR atom is the last in the stream, a parsing of such file fails.

Solution: just return immediatelly after mov_read_covr

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>