avformat/mov: Fix reading saio/saiz for clear content.

This validates that the common encryption saio/saiz atoms only appear
when the data is actually encrypted.  This also ignores those atoms
in clear content.

Found by Chrome's ClusterFuzz: http://crbug.com/850389



Signed-off-by: Jacob Trimble <modmaker@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>