Crash bug when run outside of Outpost2 folder

[DanRStevens]

I tried running the map imager on Linux (contrary to usage instructions) and got a crash:

~/code/opu/OP2MapImager$ .build/bin/op2mapimager ../opu-git-svn/GameDownload/Outpost2/trunk/
File count found for rendering: 91
--------------------------------------------------

Render initialized (May take up to 45 seconds): Ashes.map
Segmentation fault (core dumped)

Running outside of the Outpost2 folder like this seems to be unsupported, so we don't necessarily need this to work, but we should perhaps error out a little more gracefully than a crash and core dump.

[Brett208]

@DanRStevens, I was able to produce a crashing bug by trying to render a map file that could not be found on disk or in an archive. Based on the bug reported, it is too vague for me to determine if this fixes the bug you found or an unrelated bug.

Would you mind checking the branch out and seeing if it fixes your problem?

-Brett