Robust fix for union initialisation

[mpg]

See #9814 for the original problem and a discussion of possible solutions.

The original issue #9814 has been scoped down to just the simplest fix. Then #9884 adds some testing. This issue is to choose and implement a robust long-term solution. The chosen solution is likely to require (at least) ABI changes and be only applicable to development, not the LTS branches.