Fixed another PATH_MAX reliance in calc.c
Fixed another PATH_MAX reliance in calc.c

Make sure not to pass the absolute root directory to lstat() and
chdir(), as these functions can't handle long path names.

gitkaste/ncdu,rofl0r/ncdu,guns/ncdu,guns/ncdu,piyo/ncdu,FauxFaux/ncdu,rofl0r/ncdu,guns/ncdu,sunny256/ncdu,ppetr/ncdu,sunny256/ncdu,FauxFaux/ncdu,ppetr/ncdu,ppetr/ncdu,gitkaste/ncdu