Fix getenv call so that it works when lunatest is called from another ap...

getenv(3) complains about bad argument #1 when lunatest is called from another application.

I derived this patch from: KINFOO@ba897ad from pull request #3

Background:
I have a bunch of C++ code that I test with cppunit. To tie everything together I also execute lunatest from within the same C++ test executable. I think by doing so, I create a different an extra environment.

I tested on Lunix with Lua 5.1.4 and 5.1.5.