demux_mkv: fix skipping broken header elements

Fixes test4.mkv from the Matroska test file collection.

demux_mkv_open() contains a loop that reads header elements. It starts
by reading the EBML element ID with ebml_read_id(). If there is broken
data in the header, ebml_read_id() might return EBML_ID_INVALID.
However, that is not handled specially, and the code for handling
unknown tags is invoked. This reads the EBML element length in order to
skip data, which, if the EBML ID is broken, is entirely random. This
caused a seek beyond the end of the file, making the demuxer fail.

So don't skip any data if the EBML ID was invalid, and simply try to
read the next element. ebml_read_id() reads at least one byte, so the
parsing loop won't get stuck.

All in all this is rather questionable, but since this affects error
situations only, makes behavior a bit more robust (no random seeks), and
actually fixes at least one sample, it's ok.

libavformat's demuxer handled this.

demux/demux_mkv.c

@@ -1066,7 +1066,7 @@ static int read_header_element(struct demuxer *demuxer, uint32_t id,
     default:
         res = 2;
     }
-    if (!at_filepos)
+    if (!at_filepos && id != EBML_ID_INVALID)
         ebml_read_skip(s, NULL);
     return res;
 }