Explicitly check expected number of written bytes

[codecov[bot]]

Codecov Report

Patch coverage: 70.58% and project coverage change: -0.05% :warning:

Comparison is base (53b0d8b) 92.38% compared to head (75913c2) 92.34%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #236 +/- ## ========================================== - Coverage 92.38% 92.34% -0.05% ========================================== Files 25 25 Lines 2798 2807 +9 ========================================== + Hits 2585 2592 +7 - Misses 213 215 +2 ``` | [Files Changed](https://app.codecov.io/gh/JuliaIO/ProtoBuf.jl/pull/236?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/codec/vbyte.jl](https://app.codecov.io/gh/JuliaIO/ProtoBuf.jl/pull/236?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2NvZGVjL3ZieXRlLmps) | `95.45% <57.14%> (-3.36%)` | :arrow_down: | | [src/codec/encode.jl](https://app.codecov.io/gh/JuliaIO/ProtoBuf.jl/pull/236?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2NvZGVjL2VuY29kZS5qbA==) | `98.13% <80.00%> (-0.91%)` | :arrow_down: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/JuliaIO/ProtoBuf.jl/pull/236/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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