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/popupmnu.c b/src/popupmnu.c
index 09d9520..2839ea1 100644
--- a/src/popupmnu.c
+++ b/src/popupmnu.c
@@ -975,8 +975,7 @@
 	pum_undisplay();
 	while (balloon_arraysize > 0)
 	    vim_free(balloon_array[--balloon_arraysize].pum_text);
-	vim_free(balloon_array);
-	balloon_array = NULL;
+	VIM_CLEAR(balloon_array);
     }
 }