fix NaN issue in gimpoperationlevels.c

app/gegl/gimpoperationlevels.c

@@ -86,6 +86,9 @@ gimp_operation_levels_map (gdouble value,
                            gdouble low_output,
                            gdouble high_output)
 {
+  /* clamp to guard pow() against negative input */
+  value = MAX (value, low_input);
+
   /*  determine input intensity  */
   if (high_input != low_input)
     value = (value - low_input) / (high_input - low_input);