Added a missing check for division by zero.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403389

dlib/image_keypoint/surf.h

@@ -212,7 +212,9 @@ namespace dlib
         }
 
         // return the length normalized descriptor
-        des = des/length(des);
+        const double len = length(des);
+        if (len != 0)
+            des = des/len;
     }
 
 // ----------------------------------------------------------------------------------------