protect goto_instructiont::guard

[kroening]

This protects a field of goto_instructiont, in favor of using the
recommended accessor methods.

• 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

Merging #6853 (54b88f6) into develop (a9ddceb) will increase coverage by 0.00%.
The diff coverage is 70.14%.

@@           Coverage Diff            @@
##           develop    #6853   +/-   ##
========================================
  Coverage    77.78%   77.79%           
========================================
  Files         1567     1567           
  Lines       179701   179711   +10     
========================================
+ Hits        139788   139798   +10     
  Misses       39913    39913           

Impacted Files	Coverage Δ
src/analyses/cfg_dominators.h	100.00% <ø> (ø)
src/analyses/interval_analysis.cpp	0.00% <0.00%> (ø)
src/analyses/invariant_set_domain.cpp	0.00% <0.00%> (ø)
src/goto-instrument/dot.cpp	0.00% <0.00%> (ø)
src/goto-programs/remove_returns.cpp	98.15% <ø> (ø)
src/goto-programs/show_goto_functions_xml.cpp	0.00% <0.00%> (ø)
src/goto-symex/symex_goto.cpp	97.12% <ø> (ø)
src/goto-symex/symex_main.cpp	86.97% <0.00%> (ø)
src/goto-symex/solver_hardness.cpp	53.00% <11.11%> (ø)
src/goto-instrument/wmm/goto2graph.cpp	54.49% <12.50%> (ø)
... and 40 more

---

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