[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.
You can move the declaration and initialization to the place where `res` is being used (line 1633).

```c
      // HERE INIT res
      int res = SP_BAD;
      if (flags & WF_BANNED)
        res = SP_BANNED;
      else if (flags & WF_REGION) {
        // Check region.
        if ((mip->mi_lp->lp_region & (flags >> 16)) != 0)
          res = SP_OK;
        else
          res = SP_LOCAL;
      } else if (flags & WF_RARE)
        res = SP_RARE;
      else
        res = SP_OK;
```