Do not try to generate void objects

[tautschnig]

For void pointers (or structs containing void pointers) we cannot
generate "void" typed objects. Instead, generate char-typed objects as a
fall-back.

• 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.

[tautschnig]

I still need to add a regression test.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: d69fffb).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114620942

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (develop@0b8c742). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             develop    #4759   +/-   ##
==========================================
  Coverage           ?   68.46%           
==========================================
  Files              ?     1271           
  Lines              ?   104874           
  Branches           ?        0           
==========================================
  Hits               ?    71800           
  Misses             ?    33074           
  Partials           ?        0

Impacted Files	Coverage Δ
src/ansi-c/c_nondet_symbol_factory.cpp	100% <100%> (ø)

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[tautschnig]

Test added.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 8a44e73).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114716600