Try harder not to outgrow preallocated IOBuffer.data

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.06% :warning:

Comparison is base (8c751c1) 92.43% compared to head (306a637) 92.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #235 +/- ## ========================================== - Coverage 92.43% 92.38% -0.06% ========================================== Files 25 25 Lines 2817 2798 -19 ========================================== - Hits 2604 2585 -19 Misses 213 213 ``` | [Files Changed](https://app.codecov.io/gh/JuliaIO/ProtoBuf.jl/pull/235?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/codec/encode.jl](https://app.codecov.io/gh/JuliaIO/ProtoBuf.jl/pull/235?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2NvZGVjL2VuY29kZS5qbA==) | `99.03% <100.00%> (-0.09%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaIO/ProtoBuf.jl/pull/235/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO)

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