patch 8.0.1688: some macros are used without a semicolon

Problem:    Some macros are used without a semicolon, causing auto-indent to be
            wrong.
Solution:   Use the do-while(0) trick. (Ozaki Kiichi, closes #2729)
diff --git a/src/memline.c b/src/memline.c
index 5ecb32c..4be1036 100644
--- a/src/memline.c
+++ b/src/memline.c
@@ -8,8 +8,8 @@
  */
 
 /* for debugging */
-/* #define CHECK(c, s)	if (c) EMSG(s) */
-#define CHECK(c, s)
+/* #define CHECK(c, s)	do { if (c) EMSG(s); } while (0) */
+#define CHECK(c, s)	do { /**/ } while (0)
 
 /*
  * memline.c: Contains the functions for appending, deleting and changing the