cue: add missing return

Change-Id: I2f65c464b34f4ba8526af7dde0c826f3d44bf84f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2200
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/cue/ast.go b/cue/ast.go
index 7f00b4e..2a3d93c 100644
--- a/cue/ast.go
+++ b/cue/ast.go
@@ -202,7 +202,7 @@
 			switch x := e.(type) {
 			case *ast.EmitDecl:
 				// Only allowed at top-level.
-				v1.error(x, "emitting values is only allowed at top level")
+				return v1.error(x, "emitting values is only allowed at top level")
 			case *ast.Field, *ast.Alias:
 				v1.walk(e)
 			case *ast.ComprehensionDecl: