MRView: fix segfault using Up/Down keys with no image (#709)
MRView: fix segfault using Up/Down keys with no image (#709)

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