pthread_frame: don't return stale error codes after flush

pthread_frame: don't return stale error codes after flush

Consider the following sequence of events:

- open a codec without AV_CODEC_CAP_DELAY
- decode call fails with an error
- ff_thread_flush() is called
- drain packet is sent

Then the last step would make ff_thread_decode_frame() return an error,
because p->result can still be set to an error value. This is because
submit_packet returns immediately if AV_CODEC_CAP_DELAY is not set, and
no worker thread gets the chance to reset p->result, yet its value is
trusted by ff_thread_decode_frame().

Fix this by clearing the error fields on flush.