tcmur: error if both src/dst_dev are unset after CSCD parsing

A null src_dev and dst_dev pointer appears to be quite possible after
CSCD list parsing, so this error condition should be handled.

Fixes: 9c86bd0 ("tcmur: Add emulate XCOPY command support")
Signed-off-by: David Disseldorp <ddiss@suse.de>

tcmur_cmd_handler.c

@@ -1198,6 +1198,7 @@ static int xcopy_parse_target_descs(struct tcmu_device *udev,
 		}
 	}
 
+	ret = TCMU_STS_CP_TGT_DEV_NOTCONN;
 	if (xcopy->src_dev)
 		ret = xcopy_locate_udev(udev->ctx, xcopy->dst_tid_wwn,
 					&xcopy->dst_dev);