avformat/dhav: Check amount read

Prevents potential use of uninitialized data in the following
memcmp().

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/dhav.c

@@ -278,7 +278,9 @@ static int dhav_read_header(AVFormatContext *s)
     if (ret < 0)
         return ret;
 
-    avio_read(s->pb, signature, sizeof(signature));
+    ret = ffio_read_size(s->pb, signature, sizeof(signature));
+    if (ret < 0)
+        return ret;
     if (!memcmp(signature, "DAHUA", 5)) {
         avio_skip(s->pb, 0x400 - 5);
         dhav->last_good_pos = avio_tell(s->pb);