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)

src/os_vms.c

@@ -83,7 +83,7 @@ static void	set_tty(int row, int col);
 #define EXPL_ALLOC_INC 64
 
 #define EQN(S1,S2,LN) (strncmp(S1,S2,LN) == 0)
-#define SKIP_FOLLOWING_SLASHES(Str) while (Str[1] == '/') ++Str
+#define SKIP_FOLLOWING_SLASHES(Str) do { while (Str[1] == '/') ++Str; } while (0)
 
 
 /*