patch 8.0.0268: may get ml_get error when :luado deletes lines

Problem:    May get ml_get error when :luado deletes lines or switches to
            another buffer. (Nikolai Pavlov, issue #1421)
Solution:   Check the buffer and line every time.
diff --git a/src/Makefile b/src/Makefile
index 76170ec..f26a1bf 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2147,6 +2147,7 @@
 	test_langmap \
 	test_largefile \
 	test_lispwords \
+	test_lua \
 	test_man \
 	test_mapping \
 	test_marks \