security fix prevent Gajim to be run

[Yann Leboulanger <asterix@lagaule.org>]

Hi,

Your last upload about Gajim package (0.13.4-3+squeeze2) prevent Gajim 
to be run if you don't have latex / dvipng installed.

many bugs have already been opened about that:
#668710
#669100
#669105
#669106

The solution is to move this os.rename inside a "if exitcode == 0:" 
test, to be sure convertion to png was successful.

--
Yann