[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.
The else branch of this `if` has `orig_pos = pos = start_pos = curwin->w_cursor;`. You can actually remove `start_pos` from that line too [1]. Then, on line `3848` you can replace `start_pos = pos;` with `pos_T start_pos = pos;`.

[1] It wasn't considered a dead store because the `orig_pos = pos = start_pos = curwin->w_cursor` line sets and reads it, however we can see that `start_pos` is unnecessary there.