Add tests for some cases in r318298.

The first test triggers the out of bounds read of the 'left' array. It
only fails when realpath.c is compiled with '-fsanitize=address'.

The other test checks for ENOENT when running into an empty
symlink. This matches NetBSD's realpath(3) semantics. Previously,
empty symlinks were treated like ".".

Submitted by: Jan Kokemц╪ller <jan.kokemueller@gmail.com>
PR: 219154
MFC after: 2 weeks