fortify: Fix spurious warning with realpath

The length and object size arguments were swapped around for realpath.
Also add a smoke test so that any changes in this area get caught in
future.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>