fixes #1713 SP pipe_send leaks message if aio is canceled

[gdamore]

fixes #

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[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (003f055) 79.37% compared to head (759a2e9) 79.07%.

Files	Patch %	Lines
src/sp/transport/inproc/inproc.c	0.00%	2 Missing ⚠️
src/sp/transport/ipc/ipc.c	0.00%	2 Missing ⚠️
src/sp/transport/tcp/tcp.c	0.00%	2 Missing ⚠️
src/sp/transport/tls/tls.c	0.00%	2 Missing ⚠️
src/sp/transport/ws/websocket.c	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1715      +/-   ##
==========================================
- Coverage   79.37%   79.07%   -0.31%     
==========================================
  Files          89       89              
  Lines       20244    20254      +10     
==========================================
- Hits        16069    16015      -54     
- Misses       4175     4239      +64     

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[gdamore]

Waiving the coverage exception -- testing that case is rather challenging, and would really require writing a test transport framework, which is more effort than I want to spend on right now.