[libgit2-glib] git_merge_trees: ancestor_tree can be null

[Alberto Fanjul <albfan src gnome org>]

commit 8f3c2c1c7e3fdbec35024c593813d10c5ccb76de
Author: Alan Knowles <alan roojs com>
Date:   Fri Mar 15 10:08:36 2019 +0800

    git_merge_trees: ancestor_tree can be null

 libgit2-glib/ggit-repository.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/libgit2-glib/ggit-repository.c b/libgit2-glib/ggit-repository.c
index 1cdffad..fa7af84 100644
--- a/libgit2-glib/ggit-repository.c
+++ b/libgit2-glib/ggit-repository.c
@@ -3934,7 +3934,7 @@ ggit_repository_merge_trees (GgitRepository    *repository,
 
        ret = git_merge_trees (&out,
                               _ggit_native_get (repository),
-                              _ggit_native_get (ancestor_tree),
+                              ancestor_tree == NULL ? NULL : _ggit_native_get (ancestor_tree),
                               _ggit_native_get (our_tree),
                               _ggit_native_get (their_tree),
                               _ggit_merge_options_get_merge_options (merge_options));