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/spellfile.c b/src/spellfile.c
index e5cc841..e9eacbe 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -1352,8 +1352,7 @@
 	{
 	    if (c == '?' || c == '+' || c == '*')
 	    {
-		vim_free(slang->sl_comprules);
-		slang->sl_comprules = NULL;
+		VIM_CLEAR(slang->sl_comprules);
 		crp = NULL;
 	    }
 	    else