[ptx] hugin-0.6 bug, segfault changing input projection type

[Bruno Postle]

If I load an image in hugin, go to the 'camera and lens' tab and try 
to change the 'lens type' from Rectilinear to Equirectangular, I get 
this segfault:

WARN: 12:57:01.488263 (PanoramaMemento.cpp:200) getFocalLength(): 
Focal length calculations only supported with rectilinear and 
fisheye images
Segmentation fault

A workaround is to manually edit the .pto file and change the 'v' 
parameter in the 'i' lines to suit the new projection format.

-- 
Bruno