Make TrimString() safe for empty strings (fix #23)

[musicinmybrain]

When tested as described in https://github.com/Danny02/OpenCTM/issues/23, the result is now:

$ LD_LIBRARY_PATH="$PWD/lib" ./tools/ctmconv headless.off foo.ctm
Loading headless.off... 11.385 ms
Saving foo.ctm... 24.259 ms