Fix out-of-bounds access when exporting an empty mesh to CTM (fix #21)

[musicinmybrain]

When tested as described in https://github.com/Danny02/OpenCTM/issues/21, the result is now:

$ LD_LIBRARY_PATH="${PWD}/lib" ./tools/ctmconv sitting.dae sitting.ctm
Loading sitting.dae... 43.327 ms
Saving sitting.ctm... Error: CTM_INVALID_ARGUMENT

… which is a failure, but not a crash/abort.

Maybe there is a better way this could be handled with deeper understanding.