nbio: autobahn tests failures

seeing a lot of crashes on the autobahn test suite

[nbio-autobahn]: Running test case ID 9.4.4 for agent non-tls from peer tcp4:127.0.0.1:9998
[nbio-server]:   2021/08/10 12:37:47.210 [ERR] taskpool call failed: runtime error: invalid memory address or nil pointer dereference
[nbio-server]:   goroutine 7 [running]:
[nbio-server]:   github.com/lesismal/nbio/taskpool.call.func1()
[nbio-server]:          /go/src/github.com/lesismal/nbio/taskpool/caller.go:19 +0xc7
[nbio-server]:   panic(0x6bfaa0, 0x8ec800)
[nbio-server]:          /usr/local/go/src/runtime/panic.go:965 +0x1b9
[nbio-server]:   github.com/lesismal/nbio/nbhttp/websocket.(*Conn).writeFrame(0x0, 0x102, 0xc004243000, 0x1000, 0x1000, 0x0, 0x766d60, 0xc00004ace0)
[nbio-server]:          /go/src/github.com/lesismal/nbio/nbhttp/websocket/conn.go:318 +0x192
[nbio-server]:   github.com/lesismal/nbio/nbhttp/websocket.(*Conn).WriteFrame(...)
[nbio-server]:          /go/src/github.com/lesismal/nbio/nbhttp/websocket/conn.go:274
[nbio-server]:   github.com/lesismal/nbio/examples/websocket/server_autobahn.newUpgrader.func1(0x0, 0x460002, 0xc004243000, 0x1000, 0x1000)
[nbio-server]:          /go/src/github.com/lesismal/nbio/examples/websocket/server_autobahn/server.go:23 +0x88
[nbio-server]:   github.com/lesismal/nbio/nbhttp/websocket.newConn.func6(0x0, 0x50002, 0xc004243000, 0x1000, 0x1000)
[nbio-server]:          /go/src/github.com/lesismal/nbio/nbhttp/websocket/conn.go:409 +0x6d
[nbio-server]:   github.com/lesismal/nbio/nbhttp/websocket.(*Upgrader).handleDataFrame.func1()
[nbio-server]:          /go/src/github.com/lesismal/nbio/nbhttp/websocket/upgrader.go:408 +0x6d
[nbio-server]:   github.com/lesismal/nbio/nbhttp.(*Parser).Execute.func1()
[nbio-server]:          /go/src/github.com/lesismal/nbio/nbhttp/parser.go:120 +0x62
[nbio-server]:   github.com/lesismal/nbio/taskpool.call(0xc003082048)
[nbio-server]:          /go/src/github.com/lesismal/nbio/taskpool/caller.go:23 +0x5d
[nbio-server]:   github.com/lesismal/nbio/taskpool.(*fixedRunner).taskLoop(0xc000056300)
[nbio-server]:          /go/src/github.com/lesismal/nbio/taskpool/fixedpool.go:41 +0x16d
[nbio-server]:   created by github.com/lesismal/nbio/taskpool.NewFixedPool
[nbio-server]:          /go/src/github.com/lesismal/nbio/taskpool/fixedpool.go:125 +0x167
[nbio-server]:   
[nbio-server]: