core:execute: fix fork() fail handling in exec_spawn()

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.

src/core/execute.c

@@ -2056,7 +2056,7 @@ int exec_spawn(Unit *unit,
                    NULL);
         pid = fork();
         if (pid < 0)
-                return log_unit_error_errno(unit, r, "Failed to fork: %m");
+                return log_unit_error_errno(unit, errno, "Failed to fork: %m");
 
         if (pid == 0) {
                 int exit_status;