Don't set *dev in the zfs root case, it may be NULL and will correctly be

set later in the function. This fixes a potential NULL pointer dereference
found on arm64.

Obtained from:	ABT Systems Ltd
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

sys/boot/efi/loader/devicename.c

@@ -120,7 +120,6 @@ efi_parsedev(struct devdesc **dev, const char *devspec, const char **path)
 			free(idev);
 			return (err);
 		}
-		*dev = idev;
 		cp = strchr(np + 1, ':');
 	} else
 #endif