Fix physics platform behaviour regression

Lifetime checks for stored `RIDs` for collision objects assumed they had valid `object_ids`.
It turns out that some are not derived from `Object` and thus checking `ObjectDB` returns false for some valid `RIDs`.
To account for this we only perform lifetime checks on valid `object_ids`.

Author: lawnjelly

scene/3d/physics/character_body_3d.cpp

@@ -60,8 +60,13 @@ bool CharacterBody3D::move_and_slide() {
 
 			// We need to check the platform_rid object still exists before accessing.
 			// A valid RID is no guarantee that the object has not been deleted.
-			if (ObjectDB::get_instance(platform_object_id)) {
-				//this approach makes sure there is less delay between the actual body velocity and the one we saved
+
+			// We can only perform the ObjectDB lifetime check on Object derived objects.
+			// Note that physics also creates RIDs for non-Object derived objects, these cannot
+			// be lifetime checked through ObjectDB, and therefore there is a still a vulnerability
+			// to dangling RIDs (access after free) in this scenario.
+			if (platform_object_id.is_null() || ObjectDB::get_instance(platform_object_id)) {
+				// This approach makes sure there is less delay between the actual body velocity and the one we saved.
 				bs = PhysicsServer3D::get_singleton()->body_get_direct_state(platform_rid);
 			}