[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.
Hm, seems like I didn't give all of this enough thought. However, if one of the removed stores results in an access to an uninitialized variable, the static analyzer would catch this, right?

Is there a way to suppress the static analysis warnings? We'd need to keep track of "real" warnings and "won't fix" warnings somehow.