[FIX] compiler: t-key and t-ref together

Have a t-ref on a DOM node with a t-key.
Change the t-key.

Before this commit, the old block removed its element from the component's refs *after*
the new block had been mounted, meaning that in effect, the resulting
ref at the end of the whole patch was null.

After this commit, we only remove an element from the component's ref if that very same
element was indeed the ref. (otherwise it means someone else has changed the ref.)

tests/compiler/__snapshots__/misc.test.ts.snap

@@ -182,7 +182,7 @@ exports[`misc other complex template 1`] = `
 "function anonymous(app, bdom, helpers
 ) {
   let { text, createBlock, list, multi, html, toggler, comment } = bdom;
-  let { prepareList, withKey } = helpers;
+  let { prepareList, withKey, singleRefSetter } = helpers;
   const callTemplate_1 = app.getTemplate(\`LOAD_INFOS_TEMPLATE\`);
   const comp1 = app.createComponent(\`BundlesList\`, true, false, false, false);
   const comp2 = app.createComponent(\`BundlesList\`, true, false, false, false);
@@ -218,8 +218,8 @@ exports[`misc other complex template 1`] = `
   
   return function template(ctx, node, key = \\"\\") {
     const refs = this.__owl__.refs;
-    const ref1 = (el) => refs[\`search_input\`] = el;
-    const ref2 = (el) => refs[\`settings_menu\`] = el;
+    const ref1 = singleRefSetter(refs, \`search_input\`);
+    const ref2 = singleRefSetter(refs, \`settings_menu\`);
     let b2,b4,b14,b17,b22,b23,b24,b25;
     let attr1 = \`/runbot/\${ctx['project'].slug}\`;
     let txt1 = ctx['project'].name;