Replace 32-bit member variable "index" with wider local variables (#764658)

The previous commit (Fix crash reading NTFS usage when there is no
/dev/PTN entry) identified that the FileSystem member variable "index"
is too small on 64-bit machines.  Also this member variable stores no
FileSystem class information and was being used as a local variable.

Replace with local variables of the of the correct type, wide enough to
store the npos not found value.

Bug 764658 - GParted crashes when reading NTFS usage when there is no
             /dev/PTN entry