patch 7.4.717
Problem:    ":let list += list" can change a locked list.
Solution:   Check for the lock earlier. (Olaf Dabrunz)
diff --git a/src/testdir/test55.in b/src/testdir/test55.in
index 7b6f684..9e3c116 100644
--- a/src/testdir/test55.in
+++ b/src/testdir/test55.in
Binary files differ