Fix NettyRemotingClient.isChannelWritable

[zhouhailin]

remote addr isChannelWritable, if cw == null or cw.isOK() should be false.

What is the purpose of the change

fix if addr is not in channelTables.

Brief changelog

ok

Verifying this change

ok

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[coveralls]

Coverage decreased (-0.02%) to 43.163% when pulling 1d0bbb8 on zhouhailin:master into f8a67ad on apache:master.

[RongtongJin]

IMO, it is indeed a bug, but please merge into the develop branch and submit the corresponding issue. I'll close this PR first.