Crash when creating Distortion (with fix patch)

[jwernerny -2-]

The creation of a distortion file ("distortion.txt") will occasionally crash in popup.c. This is caused by not handling the possibility that the distortion file could not be created and ignoring the NULL pointer return from the "fopen(..)" call.

There is also a related issue in that "distortion.txt" is created in the current directory (more precisely, the directory navit was started from) not in specific directory. This leads to distortions being "lost" if navit is run from different directories.

The attached patch file contains fixes both for checking the file open and for always crating "distrotion.txt" in a fixed directory (I chose .navit).

[jwernerny -2-]

popup.c patch for crashing when distortion is created.

[kazer]

Patch applied in ​r3638, thank you!