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

@@ -1042,15 +1042,17 @@ static int mov_read_targa_y216(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 
 static int mov_read_ares(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 {
-    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 &&
-        atom.size > 11) {
-        avio_skip(pb, 10);
-        /* For AVID AVCI50, force width of 1440 to be able to select the correct SPS and PPS */
-        if (avio_rb16(pb) == 0xd4d)
-            codec->width = 1440;
-        return 0;
+    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 &&
+            atom.size > 11) {
+            avio_skip(pb, 10);
+            /* For AVID AVCI50, force width of 1440 to be able to select the correct SPS and PPS */
+            if (avio_rb16(pb) == 0xd4d)
+                codec->width = 1440;
+            return 0;
+        }
     }
 
     return mov_read_avid(c, pb, atom);