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/option.c b/src/option.c
index e54ba4e..21ca0fa 100644
--- a/src/option.c
+++ b/src/option.c
@@ -11735,8 +11735,7 @@
 
     if (buf == NULL)
     {
-	vim_free(*file);
-	*file = NULL;
+	VIM_CLEAR(*file);
 	return FAIL;
     }