patch 8.0.1496: clearing a pointer takes two lines

Problem:    Clearing a pointer takes two lines.
Solution:   Add VIM_CLEAR() and replace vim_clear(). (Hirohito Higashi,
            closes #2629)
diff --git a/src/normal.c b/src/normal.c
index 79e0b68..31ec363 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -1482,8 +1482,7 @@
 		{
 		    AppendToRedobuffLit(repeat_cmdline, -1);
 		    AppendToRedobuff(NL_STR);
-		    vim_free(repeat_cmdline);
-		    repeat_cmdline = NULL;
+		    VIM_CLEAR(repeat_cmdline);
 		}
 	    }
 	}