util/interval-tree: Check root for null in interval_tree_iter_first

Fix a crash in qemu-user when running

    cat /proc/self/maps

in a chroot, where /proc isn't mounted.

The problem was introduced by commit 3ce3dd8 ("util/selfmap:
Rewrite using qemu/interval-tree.h") where in open_self_maps_1() the
function read_self_maps() is called and which returns NULL if it can't
read the hosts /proc/self/maps file. Afterwards that NULL is fed into
interval_tree_iter_first() which doesn't check if the root node is NULL.

Fix it by adding a check if root is NULL and return NULL in that case.

Signed-off-by: Helge Deller <deller@gmx.de>
Fixes: 3ce3dd8 ("util/selfmap: Rewrite using qemu/interval-tree.h")
Message-Id: <ZNOsq6Z7t/eyIG/9@p100>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

util/interval-tree.c

@@ -797,7 +797,7 @@ IntervalTreeNode *interval_tree_iter_first(IntervalTreeRoot *root,
 {
     IntervalTreeNode *node, *leftmost;
 
-    if (!root->rb_root.rb_node) {
+    if (!root || !root->rb_root.rb_node) {
         return NULL;
     }