NaN gives errors in gtSegmentDiffractionBlobs (single Threshold)

Signed-off-by: Wolfgang Ludwig <wolfgang.ludwig@esrf.fr>

2_difspot/gtReadAndThreshold.m

@@ -76,7 +76,7 @@ function [img, med_value, info] = gtReadAndThreshold(ndx, parameters, thr_single
     if (isfield(seg, 'background_subtract') && (seg.background_subtract == true))
         %subtract the median of the background
         %taken from gtFullStatCondor - thanks Peter!
-        nullbb = NaN(seg.bbox(4), seg.bbox(3));
+        nullbb = zeros(seg.bbox(4), seg.bbox(3));
         imn    = gtPlaceSubImage2(nullbb, im_full, seg.bbox(1), seg.bbox(2));
         
         % can use a sub set of pixel to calculate the median, it's faster