qom: Plug memory leak in "info qom-tree"

Commit e8c9e65 "qom: Make "info qom-tree" show children sorted"
created a memory leak, because I didn't realize
object_get_canonical_path_component()'s value needs to be freed.

Reproducer:

    $ qemu-system-x86_64 -nodefaults -display none -S -monitor stdio
    QEMU 5.0.50 monitor - type 'help' for more information
    (qemu) info qom-tree

This leaks some 4500 path components, 12-13 characters on average,
i.e. roughly 100kBytes depending on the allocator.  A couple of
hundred "info qom-tree" here, a couple of hundred there, and soon
enough we're talking about real memory.

Plug the leak.

Fixes: e8c9e65
Signed-off-by: Markus Armbruster <[email protected]>
Reported-by: Reviewed-by: Li Qiang <[email protected]> [sent same patch]
Message-Id: <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>

Author: Markus Armbruster

qom/qom-hmp-cmds.c

@@ -96,8 +96,10 @@ static void print_qom_composition(Monitor *mon, Object *obj, int indent);
 
 static int qom_composition_compare(const void *a, const void *b, void *ignore)
 {
-    return g_strcmp0(a ? object_get_canonical_path_component(a) : NULL,
-                     b ? object_get_canonical_path_component(b) : NULL);
+    g_autofree char *ac = object_get_canonical_path_component(a);
+    g_autofree char *bc = object_get_canonical_path_component(b);
+
+    return g_strcmp0(ac, bc);
 }
 
 static int insert_qom_composition_child(Object *obj, void *opaque)