utlist: Add one more assertion in DL_DELETE2

Fixes #252.

src/utlist.h

@@ -711,7 +711,8 @@ do {
   assert((del)->prev != NULL);                                                                 \
   if ((del)->prev == (del)) {                                                                  \
       (head)=NULL;                                                                             \
-  } else if ((del)==(head)) {                                                                  \
+  } else if ((del) == (head)) {                                                                \
+      assert((del)->next != NULL);                                                             \
       (del)->next->prev = (del)->prev;                                                         \
       (head) = (del)->next;                                                                    \
   } else {                                                                                     \