Variadic args/va_arg: handle unexpected va_list types

[tautschnig]

Our regression test has void* lists when we'd expected void**. This, however, went unnoticed as we only tried to access the first element. Attempts to access the second element would have been misaligned, and failed an invariant as of 35cc503 (we don't handle pointer arithmetic over void pointers in the back-end anymore).

We now make sure that we always increment by size-of-void*.

Fixes: #7706

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9c7bccc) 79.66% compared to head (f755e00) 79.66%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7711   +/-   ##
========================================
  Coverage    79.66%   79.66%           
========================================
  Files         1682     1682           
  Lines       195377   195384    +7     
========================================
+ Hits        155641   155647    +6     
- Misses       39736    39737    +1     

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