Re-apply 4f33f5b: fix possible segfault in collection_unpack

The fix from #3082 was lost in merge 649aea7 (2024-04-04) and has been
missing from every release v2.9.10 .. v2.9.13. This re-applies the
same one-line guard as the original commit by @twouters.

Refs: #3082

Author: g4mm4vcl