combine_path() on relative paths with initial '.' fails under some circumstances.

Imported from https://youtrack.roxen.com/issue/PIKE-137

Reported by @grubba

Example:

Pike v8.0 release 702 running Hilfe v3.5 (Incremental Pike Frontend)
> combine_path("./../foo");
(1) Result: "foo"

The expected result is "../foo".