Do not fail an internal invariant when initialising an empty union

[tautschnig]

The C front-end should instead report a meaningful error message. The
test is based on an example originally created by CSmith and then
reduced using C-Reduce.

• 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.
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[kroening]

The message is a bit misleading -- shouldn't that index be always zero?

[kroening]

So I am thinking s.th. like "attempt to initialize an empty union"

[codecov]

Codecov Report

Merging #5664 (17ad15e) into develop (c9c3ca6) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff            @@
##           develop    #5664   +/-   ##
========================================
  Coverage    69.42%   69.42%           
========================================
  Files         1243     1243           
  Lines       100632   100636    +4     
========================================
+ Hits         69861    69866    +5     
+ Misses       30771    30770    -1     

Flag	Coverage Δ
cproversmt2	43.14% <33.33%> (-0.01%)	⬇️
regression	66.32% <66.66%> (+<0.01%)	⬆️
unit	32.26% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_initializer.cpp	76.03% <66.66%> (+0.26%)	⬆️
src/util/expr_initializer.cpp	86.51% <0.00%> (+1.12%)	⬆️

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[tautschnig]

So I am thinking s.th. like "attempt to initialize an empty union"

Thank you, I've done a slight variation on this.