#4 mars_display_roi error if no roi

[Matthew Brett]

If an empty roi is passed, it still checks for a field
in the empty roi, and gives an error
Error in ==>
/cbu/imagers/spm/spm99/toolbox/marsbar/mars_display_roi.m
On line 50 ==> warning(sprintf('ROI %d: %s - is
empty', i, label(roi)));