Fix NRE thrown from OnChannelTerminatedAsync

[AArnott]

No description provided.

[codecov]

Codecov Report

Merging #125 into master will decrease coverage by 2.17%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #125      +/-   ##
=========================================
- Coverage   88.88%   86.7%   -2.18%     
=========================================
  Files          33      33              
  Lines        2340    2340              
  Branches      436     435       -1     
=========================================
- Hits         2080    2029      -51     
- Misses        224     274      +50     
- Partials       36      37       +1

Impacted Files	Coverage Δ
src/Nerdbank.Streams/MultiplexingStream.cs	89.76% <100%> (+0.95%)	⬆️
src/Nerdbank.Streams/PipeExtensions.cs	84.09% <0%> (-15.91%)	⬇️
src/Nerdbank.Streams/MultiplexingStream.Channel.cs	78.76% <0%> (-8.85%)	⬇️

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