This commit fixes the bug in nghttp2_session_want_write.  Previously,
it may return 0 if there is pending frames after GOAWAY frame is
submitted.

To avoid the situation that nghttp2_session_want_write keeps returning
nonzero after GOAWAY and the number of active streams is 0 (e.g., keep
receiving SETTINGS or PING), nghttp2_session_mem_recv now just
swallows the input data without parsing in this case.