avformat/mov: Allow saio/saiz in clear content.

If there is a saio/saiz in clear content, we shouldn't create the
encryption index if we don't already have one.  Otherwise it will
confuse the cenc_filter.

The changed method is also used for senc atoms, but they should not
appear in clear content.

Found by Chromium's ClusterFuzz: https://crbug.com/873432



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