Extended timestamp field may be present in type 3 chunk headers

As described in Adobe’s RTMP specification dated 21 Dec 2012; also:

* RTMPY ticket #107, “Compressed headers also can have an extended
  timestamp”,
  http://web.archive.org/web/20120624031323/http://dev.rtmpy.org/ticket/107
* https://rarut.wordpress.com/2012/03/21/announcing-nginx-rtmp-module/

This resolves intermittent hangs, segfaults and crashes I was seeing when
starting ABC News 24 streams, e.g.:

rtmpdump -r rtmp://cp81899.live.edgefcs.net/live/news24-med@28772 --live

Some of the packets seemed to contain junk timestamp fields, which often
required the extended field.
Remove unused “len” variable

Apparently never used since it was introduced in Git revision b5aedcb,
Subversion revision 257.
Fix unsigned < 0 comparisons

* Previous tag size field is supposed to be unsigned (UI32 type), so cannot
  be negative
* Extended timestamp field is apparently signed (SI32 value), so check for
  negative by testing high bit
Merge remote-tracking branch 'remotes/vadmium/timestamp'
Merge remote-tracking branch 'remotes/vadmium/master'