Prevent reading thread from hanging if process has stopped

[andfoy]

Fixes #72

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 94.20290% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.66%. Comparing base (1963287) to head (c36c7cd).

Files with missing lines	Patch %	Lines
src/pty/base.rs	75.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
- Coverage   78.79%   78.66%   -0.13%     
==========================================
  Files           7        7              
  Lines        1240     1308      +68     
==========================================
+ Hits          977     1029      +52     
- Misses        263      279      +16     

Flag	Coverage Δ
unittests	78.66% <94.20%> (-0.13%)	⬇️

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