tractography: fix ROI construction when passed a 4D mask
tractography: fix ROI construction when passed a 4D mask

Bug exposed by change in ebaba50, and triggered by passing single-volume
4D mask as ROI to tckgen, resulting in:

    tckgen: [ERROR] FIXME: sizes requested for Subset adapter must be positive

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