patch 9.0.1153: build error with some compilers

Problem:    Build error with some compilers.
Solution:   Clear pointer the right way.
diff --git a/src/vim9class.c b/src/vim9class.c
index eba21ce..fd0a122 100644
--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -606,7 +606,7 @@
 		goto cleanup;
 	    for (int i = 0; i < ga_impl.ga_len; ++i)
 		cl->class_interfaces[i] = ((char_u **)ga_impl.ga_data)[i];
-	    CLEAR_POINTER(ga_impl.ga_data);
+	    VIM_CLEAR(ga_impl.ga_data);
 	    ga_impl.ga_len = 0;
 	}