Fix uninitialized variable

clang-SVN HEAD kindly provided my the info, that sm_repo maybe
uninitialized when we want to free it (If the expression in line 358 or
359/360 evaluate to true, we jump to "cleanup", where we'd use sm_repo
uninitialized).

src/submodule.c

@@ -341,7 +341,7 @@ int git_submodule_add_finalize(git_submodule *sm)
 int git_submodule_add_to_index(git_submodule *sm, int write_index)
 {
 	int error;
-	git_repository *repo, *sm_repo;
+	git_repository *repo, *sm_repo = NULL;
 	git_index *index;
 	git_buf path = GIT_BUF_INIT;
 	git_commit *head;