If pid < 0 after fork(), 0 is always returned because r =
exec_context_load_environment() has exited successfully.

This will make the caller of exec_spawn() not able to handle
the fork() error case and make systemd abort assert() possibly.

Cherry-picked from: 74129a127676e4f0edac0db4296c103e76ec6694
Resolves: #1437114