PNG export bug

[Justin Gombos <mindfuq zianet com>]

I created four diagrams and exported them PNG files.  Three of them
show a strange artifact.  I've posted the DIA and PNG files to:

http://www.zianet.com/jgombos/ns680/project/rawdata

The reston.* files have no issues, but the other three screw up on the
same icon (a VPN concentrator).  It's not purely related to the icon
though, because I have a VPN concentrator in the reston diagram and it
converts just fine.  

I'm using dia 0.92.2.