Fixed a potential bug in the hysteresis_threshold() function.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402463

dlib/image_transforms/thresholding.h

@@ -206,7 +206,7 @@ namespace dlib
         {
             for (long c = 0; c < in_img.nc(); ++c)
             {
-                typename out_image_type::type p;
+                unsigned long p;
                 assign_pixel(p,in_img[r][c]);
                 if (p >= upper_thresh)
                 {