Initialize variables in VTKReader constructor
Initialize variables in VTKReader constructor

Results in a segfault on Linux/gcc when the VTK file does not
contain any points.  Initialize number_of_lines and
number_of_line_indices to 0.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3