Fix union initialization

[tautschnig]

The attached test shows that we could reach recursive initialisation of
unions with a byte_update instead of a union expression. The test was
constructed using creduce, starting from code included in the Linux
kernel.

• 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 #5967 (29b284f) into develop (7663a57) will decrease coverage by 0.02%.
The diff coverage is 58.18%.

@@             Coverage Diff             @@
##           develop    #5967      +/-   ##
===========================================
- Coverage    74.30%   74.27%   -0.03%     
===========================================
  Files         1444     1444              
  Lines       157453   157460       +7     
===========================================
- Hits        116995   116953      -42     
- Misses       40458    40507      +49     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_conv.h	100.00% <ø> (ø)
src/ansi-c/c_typecheck_initializer.cpp	61.61% <44.44%> (-12.97%)	⬇️
src/solvers/smt2/smt2_conv.cpp	60.38% <52.94%> (ø)
src/solvers/smt2/smt2_solver.cpp	83.11% <65.51%> (ø)
src/util/config.cpp	55.42% <0.00%> (+3.03%)	⬆️
src/util/config.h	62.50% <0.00%> (+12.50%)	⬆️

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