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/873432Signed-off-by: Jacob Trimble <modmaker@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -5845,6 +5845,9 @@ static int get_current_encryption_info(MOVContext *c, MOVEncryptionIndex **encry
         *sc = st->priv_data;
 
         if (!frag_stream_info->encryption_index) {
+            // If this stream isn't encrypted, don't create the index.
+            if (!(*sc)->cenc.default_encrypted_sample)
+                return 0;
             frag_stream_info->encryption_index = av_mallocz(sizeof(*frag_stream_info->encryption_index));
             if (!frag_stream_info->encryption_index)
                 return AVERROR(ENOMEM);
@@ -5860,6 +5863,9 @@ static int get_current_encryption_info(MOVContext *c, MOVEncryptionIndex **encry
         *sc = st->priv_data;
 
         if (!(*sc)->cenc.encryption_index) {
+            // If this stream isn't encrypted, don't create the index.
+            if (!(*sc)->cenc.default_encrypted_sample)
+                return 0;
             (*sc)->cenc.encryption_index = av_mallocz(sizeof(*frag_stream_info->encryption_index));
             if (!(*sc)->cenc.encryption_index)
                 return AVERROR(ENOMEM);