realpath.c fails with special test cases

[dfhawthorne]

The following test cases all fail:

1. '/fred/.'
2. '/fred/..'
3. '/fred/fred'
4. '//fred/.'
5. '//fred/..'
6. '//fred/fred'

realpath returns the error, No such file or directory, in all of these cases.

[dfhawthorne]

Resolved by adding function validateDirectory.