Compiler: Ensure that foreach always generates a node.

Fixes LysLysKOM 19705902:
| 19705902 2012-05-22 12:22 /4 rader/ Martin Nilsson (Opera Mini - AFK!)
| Mottagare: Henrik Grubbström (Lysator) <11101>
| Mottagare: Martin Nilsson (Opera Mini - AFK!) <14470>
|     Mottaget: 2012-05-22 12:22
| Ärende: Parser bug
| ------------------------------------------------------------
|
| Pike v7.9 release 5 running Hilfe v3.5 (Incremental Pike Frontend)
| > foreach(f, mapping(string:string));
| Segmentation fault
| (19705902) /Martin Nilsson (Opera Mini - AFK!)/-----