lavf/mov: make invalid m{d,v}hd time_scale default to 1 instead of erroring out

Some samples have their metadata track time_scale incorrectly set to 0
and the check introduced by a398f054fdb9b0f0b5a91c231fba6ce014143f71
prevents playback of those samples. Setting the time_scale to 1 fixes
playback.
