[FIX] qweb: do not crash in some rare t-call situation

Before this commit, QWeb crashed when it had to deal with a t-call with
multiple sub nodes on the same line, and one text node:

<t t-call="SomeTemplate">
  <span>hey</span> <span>hey</span>
</t>

This was because we need to compile the content to be able to extract
all possible t-set t-value statements. But doing so means that we had a
multiple root templates, which caused a crash in this case.

Solving this issue is simple: the sub template is compiled with the flag
allowMultipleRoots set to true.

src/qweb/base_directives.ts

@@ -200,6 +200,7 @@ QWeb.addDirective({
     // extract variables from nodecopy
     const tempCtx = new Context();
     tempCtx.nextID = ctx.rootContext.nextID;
+    tempCtx.allowMultipleRoots = true;
     qweb._compileNode(nodeCopy, tempCtx);
     const vars = Object.assign({}, ctx.variables, tempCtx.variables);
     ctx.rootContext.nextID = tempCtx.nextID;