core-graphics: Assign a value to a _ variable

Compiler thinks that we need to use the value, so we use this workaroud
to get rid of a warning.

core-graphics/src/event.rs

@@ -505,7 +505,7 @@ impl<'tap_life> CGEventTap<'tap_life> {
                     callback_ref: Box::from_raw(cbr),
                 })
             } else {
-                Box::from_raw(cbr);
+                let _ = Box::from_raw(cbr);
                 Err(())
             }
         }