remove redundant if statements in avl tree delete_generic()

[mvandenhoek]

This was discovered whilst trying to improve code coverage. The issue is with the following code:

cyclonedds/src/ddsrt/src/avl.c

Lines 471 to 528 in 85261c1

	if (node->cs[0] == NULL)
	{
	if (node->cs[1]) {
	node->cs[1]->parent = node->parent;
	}
	*pnode = node->cs[1];
	whence = node->parent;
	}
	else if (node->cs[1] == NULL)
	{
	node->cs[0]->parent = node->parent;
	*pnode = node->cs[0];
	whence = node->parent;
	}
	else
	{
	ddsrt_avl_node_t *subst;
	assert (node->cs[0] != NULL);
	assert (node->cs[1] != NULL);
	/* Use predecessor as substitute */
	if (path) {
	path->p.pnode[++path->p.depth] = &node->cs[0];
	}
	subst = node->cs[0];
	if (subst->cs[1] == NULL) {
	whence = subst;
	} else {
	do {
	if (path) {
	path->p.pnode[++path->p.depth] = &subst->cs[1];
	}
	subst = subst->cs[1];
	} while (subst->cs[1]);
	whence = subst->parent;
	whence->cs[1] = subst->cs[0];
	if (whence->cs[1]) {
	whence->cs[1]->parent = whence;
	}
	subst->cs[0] = node->cs[0];
	if (subst->cs[0]) {
	subst->cs[0]->parent = subst;
	}
	if (path) {
	path->p.pnode[path->p.pnodeidx+1] = &subst->cs[0];
	}
	}
	subst->parent = node->parent;
	subst->height = node->height;
	subst->cs[1] = node->cs[1];
	if (subst->cs[1]) {
	subst->cs[1]->parent = subst;
	}
	*pnode = subst;
	}

If you go into the else of line 485, then proceed past line 506, you have a situation like in the depicted below:

             node <-- delete target
            /    \
           /      \
        whence     subtree
        /   \
 subtree    subst
           /
          ? 

The if and if else preceding the else of line 485 guarantee node->cs[0] != NULL and node->cs[1] != NULL.
Thus if (subst->cs[0]) { can be scrapped.
Likewise, if (subst->cs[1]) { can also be scrapped.

[eboasson]

Indeed, all nodes are unique and so node, subst and whence are do not alias. Thus the value of node->cs[{0,1}] does not changed and the new values of subst->cs[{0,1}] cannot be null pointers.