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/main.c b/src/main.c
index 971590a..78a3018 100644
--- a/src/main.c
+++ b/src/main.c
@@ -3951,8 +3951,7 @@
 		{
 		    /* Output error from remote */
 		    mch_errmsg((char *)res);
-		    vim_free(res);
-		    res = NULL;
+		    VIM_CLEAR(res);
 		}
 		mch_errmsg(_(": Send expression failed.\n"));
 	    }