cue/ast: don't require ellipsis to be at end of struct.

This is a first step in generally allowing this, according to
the modified step.

The prevents a crash in case the input is incorrect.

Change-Id: Ia3bcd697548684dcf09367e7fa51a19a758838c9
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6323
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/ast.go

@@ -204,7 +204,7 @@ func (v *astVisitor) walk(astNode ast.Node) (ret value) {
 			astState: v.astState,
 			object:   obj,
 		}
-		for i, e := range n.Decls {
+		for _, e := range n.Decls {
 			switch x := e.(type) {
 			case *ast.EmbedDecl:
 				if v1.object.emit == nil {
@@ -213,9 +213,7 @@ func (v *astVisitor) walk(astNode ast.Node) (ret value) {
 					v1.object.emit = mkBin(v.ctx(), token.NoPos, opUnify, v1.object.emit, v1.walk(x.Expr))
 				}
 			case *ast.Ellipsis:
-				if i != len(n.Decls)-1 {
-					return v1.walk(x.Type) // Generate an error
-				}
+				// handled elsewhere
 
 			default:
 				v1.walk(e)