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.