dwi2fod: fix error when volumes not assigned to shells
dwi2fod: fix error when volumes not assigned to shells

This was due to uninitialised rows in the transform matrix when some of
the volumes were left out.

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