bv_pointerst: else case is just for byte updates

[tautschnig]

We should not treat all other expressions as byte updates. Expressions
other than those handled in the if/else if chain should result in
failing conversion.

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[codecov]

Codecov Report

Merging #6138 (d73135a) into develop (74e4365) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@             Coverage Diff             @@
##           develop    #6138      +/-   ##
===========================================
- Coverage    75.96%   75.96%   -0.01%     
===========================================
  Files         1508     1508              
  Lines       163292   163295       +3     
===========================================
+ Hits        124052   124054       +2     
- Misses       39240    39241       +1     

Impacted Files	Coverage Δ
src/solvers/flattening/bv_pointers.cpp	82.56% <66.66%> (-0.11%)	⬇️

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[TGWDB]

This looks nice and a good way to catch errors. Is it possible to actually reach this code path? If so can you create a regression test that catches it?

[martin-cs]

I think @TGWDB is correct to request a test but I think this is a rare code path and may only be taken by other things that also need to be fixed so I am willing to merge without one if @tautschnig says that adding one would be testing for incorrect or buggy behaviour.