avformat/dhav: Fix incorrect non-DHAV chunk skipping logic

DAV files may contain a variable length padding in between chunks
filled with 0xff bytes. The current skipping logic is incorrect as it
may skip over DHAV chunks not appearing sequentially in the file.

We now look for the 'DHAV' tag using a byte-by-byte search in order
to handle such situations. Also the dhav->last_good_pos field will
not be updated while skipping unrecognized data.

libavformat/dhav.c

@@ -173,18 +173,9 @@ static int read_chunk(AVFormatContext *s)
     if (avio_feof(s->pb))
         return AVERROR_EOF;
 
-    if (avio_rl32(s->pb) != MKTAG('D','H','A','V') && dhav->last_good_pos < INT64_MAX - 0x8000) {
-        dhav->last_good_pos += 0x8000;
-        avio_seek(s->pb, dhav->last_good_pos, SEEK_SET);
-
-        while (avio_rl32(s->pb) != MKTAG('D','H','A','V')) {
-            if (avio_feof(s->pb) || dhav->last_good_pos >= INT64_MAX - 0x8000)
-                return AVERROR_EOF;
-            dhav->last_good_pos += 0x8000;
-            ret = avio_skip(s->pb, 0x8000 - 4);
-            if (ret < 0)
-                return ret;
-        }
+    while (avio_r8(s->pb) != 'D' || avio_r8(s->pb) != 'H' || avio_r8(s->pb) != 'A' || avio_r8(s->pb) != 'V') {
+        if (avio_feof(s->pb))
+            return AVERROR_EOF;
     }
 
     start = avio_tell(s->pb) - 4;