patch 8.2.0173: build fails with old compiler

Problem:    Build fails with old compiler.
Solution:   Do not use anonymous unions. (John Marriott)
diff --git a/src/if_mzsch.c b/src/if_mzsch.c
index e31d0e1..0ee922d 100644
--- a/src/if_mzsch.c
+++ b/src/if_mzsch.c
@@ -3044,7 +3044,7 @@
 	    MZ_GC_VAR_IN_REG(0, obj);
 	    MZ_GC_REG();
 
-	    curr = list->lv_last;
+	    curr = list->lv_u.mat.lv_last;
 	    obj = vim_to_mzscheme_impl(&curr->li_tv, depth + 1, visited);
 	    result = scheme_make_pair(obj, scheme_null);
 	    MZ_GC_CHECK();