vfio: Fix smatch errors in vfio_combine_iova_ranges()

smatch reports:

vfio_combine_iova_ranges() error: uninitialized symbol 'last'.
vfio_combine_iova_ranges() error: potentially dereferencing uninitialized 'comb_end'.
vfio_combine_iova_ranges() error: potentially dereferencing uninitialized 'comb_start'.

These errors are only reachable via invalid input, in the case of
@last when we receive an empty rb-tree or for @comb_{start,end} if the
rb-tree is empty or otherwise fails to produce a second node that
reduces the gap.  Add tests with warnings for these cases.

Reported-by: Cong Liu <liucong2@kylinos.cn>
Link: https://lore.kernel.org/all/20230920095532.88135-1-liucong2@kylinos.cn
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Link: https://lore.kernel.org/r/20231002224325.3150842-1-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: Alex Williamson

drivers/vfio/vfio_main.c

@@ -946,6 +946,11 @@ void vfio_combine_iova_ranges(struct rb_root_cached *root, u32 cur_nodes,
 		unsigned long last;
 
 		comb_start = interval_tree_iter_first(root, 0, ULONG_MAX);
+
+		/* Empty list */
+		if (WARN_ON_ONCE(!comb_start))
+			return;
+
 		curr = comb_start;
 		while (curr) {
 			last = curr->last;
@@ -975,6 +980,11 @@ void vfio_combine_iova_ranges(struct rb_root_cached *root, u32 cur_nodes,
 			prev = curr;
 			curr = interval_tree_iter_next(curr, 0, ULONG_MAX);
 		}
+
+		/* Empty list or no nodes to combine */
+		if (WARN_ON_ONCE(min_gap == ULONG_MAX))
+			break;
+
 		comb_start->last = comb_end->last;
 		interval_tree_remove(comb_end, root);
 		cur_nodes--;