crash if inputfile name length is less than 4 chars

[ferkulat]

Calling csv2xls with:

csv2xls inp

ends with

terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::substr
Aborted (core dumped)

This works

csv2xls -o l.xls inp

This does not work again

csv2xls -o l.xl inp