[list] Add sanity checks after list-adding functions

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/list.h

 #define list_add( new, head ) do {				\
 	list_check ( (head) );					\
 	extern_list_add ( (new), (head) );			\
+	list_check ( (head) );					\
+	list_check ( (new) );					\
 	} while ( 0 )
 static inline void inline_list_add ( struct list_head *new,
 				     struct list_head *head ) {
 #define list_add_tail( new, head ) do {				\
 	list_check ( (head) );					\
 	extern_list_add_tail ( (new), (head) );			\
+	list_check ( (head) );					\
+	list_check ( (new) );					\
 	} while ( 0 )
 static inline void inline_list_add_tail ( struct list_head *new,
 					  struct list_head *head ) {