PNG scanning code change

Hello,
I changed the current implementation of the PNG scanner (which was mostly a
PNG validator) into an exploit seeker with PR #73. I added the detection
for a couple of exploits I knew (but this thing can be totally expanded); I
actually don't know if I used correctly the cli_append_virus function as I
made up the names of the heuristics by pointing out which CVE is associated
to the detected vulnerability, but hopefully I did this right.

Also, now it scans the overlay (if present).
_______________________________________________
http://lurker.clamav.net/list/clamav-devel.html
Please submit your patches to our Bugzilla: http://bugs.clamav.net

http://www.clamav.net/contact.html#ml