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

lib/libzutil/os/linux/zutil_device_path_os.c

@@ -346,7 +346,7 @@ end:
 	free(tmp);
 	free(realp);
 
-	if (!path) {
+	if (!path && first_path) {
 		/*
 		 * None of the underlying paths had a link back to their
 		 * enclosure devices.  Throw up out hands and return the first