Avoid STOP_SENDING for unknown stream type in case fin is set.

[moritzbuhl]

My HTTP3 client receives a grease frame from quiche and it includes a fin.

STREAM id=15 fin=1 off=0 len=26 dir=Unidirectional origin=Server-initiated
    Frame Type: STREAM (0x000000000000000f)
    Stream ID: 15
    Offset: 0
    Length: 26
    Stream Data: f739d7873c274c684752454153452069732074686520776f7264

after running gdb I see that nghttp3 does nghttp3_conn_read_uni and then identifies stream->type is NGHTTP3_STREAM_TYPE_UNKNOWN and proceeds with conn_call_stop_sending.

Looking at RFC9114 Reserved Stream Types

Endpoints MUST NOT consider these streams to have any meaning upon receipt.

If I understand this right, maybe it is never a good idea to send STOP_SENDING?

[tatsuhiro-t]

Thank you for PR. Merged now.