avoid spurious bug with eager gc

[codecov[bot]]

Codecov Report

Base: 84.09% // Head: 84.09% // No change to project coverage :thumbsup:

Coverage data is based on head (f8f3aae) compared to base (00e1931). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #435 +/- ## ======================================= Coverage 84.09% 84.09% ======================================= Files 30 30 Lines 4113 4113 ======================================= Hits 3459 3459 Misses 654 654 ``` | [Impacted Files](https://codecov.io/gh/JuliaIO/JLD2.jl/pull/435?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/data/writing\_datatypes.jl](https://codecov.io/gh/JuliaIO/JLD2.jl/pull/435/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2RhdGEvd3JpdGluZ19kYXRhdHlwZXMuamw=) | `96.84% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO)

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[oameye]

When will this be merged?