Bug fix: When moving from C style pointer based strings to the C++ string

Bug fix: When moving from C style pointer based strings to the C++ string
class the memebr initialization list was not updated accordingly.
The member fDirectory and fFilename was initialized to zero wich
is fine for a C pointerbased string but when used to initialize a string
with the statement fDirctory(0) it is intepret as an attemt to initialize
a string object with a C character string starting at 0.
The error cuase a runtime error of aliroot, but is now fixed.