[ptx] segfault in auto preview

[Rich]

i think i mentioned this some time ago, but there was no response.
test scenario :
create a new project, add two images. add a single control point, 
optimize, apply.
open preview; enable auto mode. click on one of the image buttons three 
times (do it relatively fast). hugin segfaults.
reproducible always. hugin 0.5, slackware linux.

i could provide debugging output if needed or test anything required to 
pin down this nasty problem.
-- 
  Rich