WPrefs: fix memory leak when editing a delay for the dock (Coverity #50156)

As reported by Coverity, the return string of WMGetTextFieldText is
malloced so it needs to be freed.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

WPrefs.app/Docks.c

@@ -120,6 +120,7 @@ static void autoDelayChanged(void *observerData, WMNotification *notification)
 		}
 		char *value = WMGetTextFieldText(anAutoDelayT);
 		adjustButtonSelectionBasedOnValue(panel, row, value);
+		free(value);
 		return;
 	}
 }