Piped process: fix off-by-one error

[tautschnig]

The NULL terminator previously was an out-of-bounds write. This fixes the test failure seen in
https://github.com/diffblue/cbmc/actions/runs/5112785459/jobs/9191230589?pr=7395, which was locally reproducible. Out-of-bounds write found by valgrind via

regression/cbmc/dynamic_sizeof1$ valgrind \
  ../../../build-coverage/bin/cbmc main.c \
  --incremental-smt2-solver 'z3 --smt2 -in'

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02 ⚠️

Comparison is base (d5e13f1) 78.54% compared to head (c47e940) 78.53%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7738      +/-   ##
===========================================
- Coverage    78.54%   78.53%   -0.02%     
===========================================
  Files         1688     1688              
  Lines       193001   193001              
===========================================
- Hits        151599   151565      -34     
- Misses       41402    41436      +34     

Impacted Files	Coverage Δ
src/util/piped_process.cpp	81.92% <100.00%> (ø)

... and 1 file with indirect coverage changes

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