Line numbers wrong due to shared nodes.

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=665

Reported by Martin Stjernholm mast@roxen.com

In a certain situation in Roxen I encountered a place where the line number info gets very wrong. To reproduce:

1. Get a Roxen 2.2 from about 21:00 November 6.
2. Apply the attached patch to server/etc/modules/RXML.pmod/module.pmod.
3. Create a normal server with a filesystem and an rxml parser.
4. Create a file containing "&form.var;" and request it.

This will produce a Pike backtrace where the line number in the top entry is 374, which is way outside the offending function rxml_index.

The line number gets correct if the freeze_node function in las.c is defeated.