Add asserting element's popover invoker is null in the "show popover"…

… algorithm

Fixes whatwg#9383.

source

@@ -82437,6 +82437,8 @@ dictionary <dfn dictionary>DragEventInit</dfn> : <span>MouseEventInit</span> {
   <ol>
    <li><p>Let <var>document</var> be <var>element</var>'s <span>node document</span>.</p></li>
 
+   <li><p><span>Assert</span>: <var>element</var>'s <span>popover invoker</span> is null.</p></li>
+
    <li><p>Set <var>element</var>'s <span>popover invoker</span> to <var>invoker</var>.</p></li>
 
    <li><p>If the result of running <span>check popover validity</span> given <var>element</var>,
@@ -134968,6 +134970,7 @@ INSERT INTERFACES HERE
   Mike Shaver,
   Mikko Rantalainen,
   Mingye Wang,
+  Mirko Brodesser,
   Mohamed Zergaoui<!-- Innovimax SARL -->,
   Mohammad Al Houssami,
   Mohammad Reza Zakerinasab,