avformat/aadec: Check for scanf() failure

Fixes: use of uninitialized variables
Fixes: blank.aa
Found-by: Chamal De Silva <chamal.desilva@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ed188f6d)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/aadec.c

@@ -78,6 +78,7 @@ static int aa_read_header(AVFormatContext *s)
     AADemuxContext *c = s->priv_data;
     AVIOContext *pb = s->pb;
     AVStream *st;
+    int ret;
 
     /* parse .aa header */
     avio_skip(pb, 4); // file size
@@ -121,7 +122,10 @@ static int aa_read_header(AVFormatContext *s)
         }
         if (!strcmp(key, "HeaderKey")) { // this looks like "1234567890 1234567890 1234567890 1234567890"
             av_log(s, AV_LOG_DEBUG, "HeaderKey is <%s>\n", val);
-            sscanf(val, "%u%u%u%u", &header_key_part[0], &header_key_part[1], &header_key_part[2], &header_key_part[3]);
+            ret = sscanf(val, "%u%u%u%u", &header_key_part[0], &header_key_part[1], &header_key_part[2], &header_key_part[3]);
+            if (ret != 4)
+                return AVERROR_INVALIDDATA;
+
             for (idx = 0; idx < 4; idx++) {
                 AV_WB32(&header_key[idx * 4], header_key_part[idx]); // convert each part to BE!
             }