Fix thread.posix.c

- Fix memory leak when thread_spawn fails
- Remove incorrect assertion (pthread_t in general case is neither scalar
  nor non-zero)

src/common/util/thread.posix.c

@@ -76,14 +76,15 @@ thread_spawn(ThreadEntry entry, ThreadResult error_result)
 	pthread_t handle;
 	int error;
 	if (!new_uninitialized(&th)) {
+		free(native_arg);
 		return THREAD_NONE;
 	}
-	error = pthread_create(&handle, NULL, &run_entry, native_arg);
+	error = pthread_create(&handle, NULL, &run_entry, native_arg);  // calls free(native_arg) if error == 0
 	if (error) {
 		errno = error;
+		free(native_arg);
 		return delete_handle(th);
 	}
-	assert(handle && "System allows 0 as thread id");
 	initialize_handle(&th, handle);
 	return th;
 }