Fix assert in ravl_node_rank_difference_parent()

Fix assert in ravl_node_rank_difference_parent().
ravl_node_rank() do not have to be positive - it can return (-1),
so the assert has to be corrected.

For example:
if ravl_node_rank(p) = 0 and ravl_node_rank(n) = -1, then rv = 1.

Ref: #717

Signed-off-by: Lukasz Dorau <[email protected]>

Author: ldorau

src/ravl/ravl.c

@@ -255,7 +255,8 @@ static int ravl_node_rank_difference_parent(struct ravl_node *p,
                                             struct ravl_node *n) {
     int rv = ravl_node_rank(p) - ravl_node_rank(n);
     // assert to check integer overflow
-    assert(rv < ravl_node_rank(p));
+    // ravl_node_rank(x) is >= -1
+    assert(rv <= ravl_node_rank(p) + 1);
     return rv;
 }