Changed the way we guard against length zero descriptor vectors slightly.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403430

dlib/image_keypoint/surf.h

@@ -211,9 +211,9 @@ namespace dlib
             }
         }
 
-        // return the length normalized descriptor
-        const double len = length(des);
-        if (len != 0)
+        // Return the length normalized descriptor.  Add a small number
+        // to guard against division by zero.
+        const double len = length(des) + 1e-7;
         des = des/len;
     }