Normative: correct locator handling of "max depth" (#611)

Author: jugglinmike

index.bs

@@ -3055,7 +3055,9 @@ and |session|:
 
     1. Otherwise, perform the following steps:
 
-      1. Set |max depth| to |max depth| - 1.
+      1. If |max depth| is not null:
+
+        1. Set |max depth| to |max depth| - 1.
 
       1. Let |child node matches| be the result of [=locate nodes using inner text=]
          with |context target|, |child nodes|, |selector|, |max depth|,