libzutil: fix dm_get_underlying_path() return if not a DM device

For example, this would happily return "/dev/(null)" for /dev/sda1

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11935