The only reason videotoolbox wouldn't produce frames is if the data fed
to it was invalid, so returning AVERROR_INVALIDDATA makes sense here.

Further, it means AVERROR_EXTERNAL can be used in further commits to signal
fatal VideoToolbox errors, letting the user know that they need to fallback to
another decoder.
Signed-off-by: Aman Gupta <aman@tmm1.net>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>