Don’t try to read a path that wasn’t supplied in the command line options.

Hi,

When the options "-cfg" or "-data" are provided at the end of the command line without a path, M.A.R.S. segfaults and dies.

Attached patch fixes the problem by preventing an off-by-one access to argv[] and display a non-fatal error message in these cases.

Best wishes,

-- 
Sylvain Boilard