x/crypto/openpgp: infinite loop on compression quines

[kbsriram]

A carefully crafted packet can cause the ReadMessage function to push an infinite number of io.Readers within the packet reader.

An example packet created by Taylor R. Campbell can be found at:
http://mumble.net/~campbell/misc/pgp-quine/
and additional details can be found from this related issue for GnuPG:
https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2013-4402

[ianlancetaylor]

CC @agl

[ebfe]

This has been fixed in golang/crypto@24ffb5feb