avformat/mov: Init ref_sc / ref_st to NULL

This is more robust in case some change or corner case causes them to be
dereferenced before being set

Fixes CID1396274, CID1396275

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -4338,8 +4338,8 @@ static int mov_read_sidx(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     uint8_t version;
     unsigned i, track_id;
     AVStream *st = NULL;
-    AVStream *ref_st;
-    MOVStreamContext *sc, *ref_sc;
+    AVStream *ref_st = NULL;
+    MOVStreamContext *sc, *ref_sc = NULL;
     MOVFragmentIndex *index = NULL;
     MOVFragmentIndex **tmp;
     AVRational timescale;