libavformat/mov: Fix NULL-dereference read for some encrypted content.

When reading frames, we need to use the fragment for the correct
stream.  Sometimes the "current" fragment is not the same as the one
the frame is for.

Found by Chromium's ClusterFuzz:
https://crbug.com/906392 and https://crbug.com/915524Signed-off-by: Jacob Trimble <modmaker@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -6557,14 +6557,14 @@ static int cenc_decrypt(MOVContext *c, MOVStreamContext *sc, AVEncryptionInfo *s
     return 0;
 }
 
-static int cenc_filter(MOVContext *mov, MOVStreamContext *sc, AVPacket *pkt, int current_index)
+static int cenc_filter(MOVContext *mov, AVStream* st, MOVStreamContext *sc, AVPacket *pkt, int current_index)
 {
     MOVFragmentStreamInfo *frag_stream_info;
     MOVEncryptionIndex *encryption_index;
     AVEncryptionInfo *encrypted_sample;
     int encrypted_index, ret;
 
-    frag_stream_info = get_current_frag_stream_info(&mov->frag_index);
+    frag_stream_info = get_frag_stream_info(&mov->frag_index, mov->frag_index.current, st->id);
     encrypted_index = current_index;
     encryption_index = NULL;
     if (frag_stream_info) {
@@ -7794,7 +7794,7 @@ static int mov_read_packet(AVFormatContext *s, AVPacket *pkt)
     if (mov->aax_mode)
         aax_filter(pkt->data, pkt->size, mov);
 
-    ret = cenc_filter(mov, sc, pkt, current_index);
+    ret = cenc_filter(mov, st, sc, pkt, current_index);
     if (ret < 0)
         return ret;