treebuilder: prevent git_treebuilder_free() from segfaulting when being passed a NULL treebuilder

src/tree.c

@@ -634,6 +634,9 @@ void git_treebuilder_clear(git_treebuilder *bld)
 
 void git_treebuilder_free(git_treebuilder *bld)
 {
+	if (bld == NULL)
+		return;
+
 	git_treebuilder_clear(bld);
 	git_vector_free(&bld->entries);
 	git__free(bld);