Fix various errors in list_destroy()

As reported by cppcheck:

    src/list.c:45:8: style: Redundant initialization for 'data'. The
    initialized value is overwritten before it is read.
    [redundantInitialization]
      data = NULL;
       	   ^
    src/list.c:43:15: note: data is initialized
      void * data = list_pop_front(list);
              	  ^
    src/list.c:45:8: note: data is overwritten
      data = NULL;
       	   ^

src/list.c

@@ -40,9 +40,7 @@ void list_destroy(List_t * list){
 	// sans détruire les données dedans ?	
 	while(!list_is_empty(list)){
 		pDEBUG("destroying front cell\n");
-		void * data = list_pop_front(list);
+		list_pop_front(list);
-		pDEBUG("set data to NULL\n");
-		data = NULL;
 	}
 	free(list);
 }