remote: don't check for upstream on an anonymous remote

If the remote is anonymous, then we cannot check for any configuration,
as there is no name. Check for this before we try to use the name, which
may be a NULL pointer.

This fixes #2697.

src/remote.c

@@ -959,6 +959,7 @@ static int remote_head_for_ref(git_remote_head **out, git_remote *remote, git_re
 	}
 
 	if ((!git_reference__is_branch(ref_name)) ||
+	    !git_remote_name(remote) ||
 	    (error = git_branch_upstream_remote(&upstream_remote, repo, ref_name)) ||
 	    git__strcmp(git_remote_name(remote), git_buf_cstr(&upstream_remote)) ||
 	    (error = git_branch_upstream_name(&upstream_name, repo, ref_name)) < 0 ||

tests/network/remote/remotes.c

@@ -535,6 +535,16 @@ static int remote_single_branch(git_remote **out, git_repository *repo, const ch
 	return 0;
 }
 
+void test_network_remote_remotes__fetch_from_anonymous(void)
+{
+	git_remote *remote;
+
+	cl_git_pass(git_remote_create_anonymous(&remote, _repo, cl_fixture("testrepo.git"),
+						"refs/heads/*:refs/other/*"));
+	cl_git_pass(git_remote_fetch(remote, NULL, NULL, NULL));
+	git_remote_free(remote);
+}
+
 void test_network_remote_remotes__single_branch(void)
 {
 	git_clone_options opts = GIT_CLONE_OPTIONS_INIT;