avformat/mov: Check that we have a stream before accessing it in mov_read_ares()

Fixes out of array read
Fixes: signal_sigsegv_6f1855_3910_avid_test_alpha.mov

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -1042,6 +1042,7 @@ static int mov_read_targa_y216(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 
 static int mov_read_ares(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 {
+    if (c->fc->nb_streams >= 1) {
         AVCodecContext *codec = c->fc->streams[c->fc->nb_streams-1]->codec;
         if (codec->codec_tag == MKTAG('A', 'V', 'i', 'n') &&
             codec->codec_id == AV_CODEC_ID_H264 &&
@@ -1052,6 +1053,7 @@ static int mov_read_ares(MOVContext *c, AVIOContext *pb, MOVAtom atom)
                 codec->width = 1440;
             return 0;
         }
+    }
 
     return mov_read_avid(c, pb, atom);
 }