Restore namespace after symbolic execution [blocks: #3976]

[tautschnig]

This was already fixed in d475abc, but 685937a introduced additional
returns. Now use an approach that will always work, even when returning via
exceptions being thrown.

• Each commit message has a non-empty body, explaining why the change was made.
• 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.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 58e0f10).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98730024

[peterschrammel]

Maybe add a one-liner explaining what this aims to achieve.

[tautschnig]

Done!

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 503e42f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98934307
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 23196af).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99024802