[RFC] Clang analyzer: fix dead stores

Remove dead assignments as pointed out by [static analysis](http://neovim.org/doc/build-reports/clang/). [Here](https://fwalch.github.io/doc-ci/build-reports/clang) is a static analysis report that includes the changes of this PR.

I didn't fix the [dead assignment in garray.c](http://neovim.org/doc/build-reports/clang/report-c565a2.html#EndPath), because otherwise the build would fail with `error: ignoring return value of ‘xstpcpy’, declared with attribute warn_unused_result`, and I didn't know what to do about that.
At first impression, I thought this would better stay, but after checking the code I see that it doesn't make sense to update `len`. Doing this may actually be more confusing.