#47 Disk full --> segfault

[RanTalbott]

This problem occurred running 0.17 on Linux.

If the volume containing the map data directory fills up while downloading/compiling map data, roadnav will segfault and die without identifying the error to the user. Because of the speed with which this happened, I was unable to tell whether the failure was in the code that was building the files, or other code trying to use corrupted data.