[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 declare and initialize `width` close to the `if` that sets it (line ~7799).

```c
  validate_virtcol_win(wp);
  if (       redraw_cmdline
             || always
             || wp->w_cursor.lnum != wp->w_ru_cursor.lnum
             || wp->w_cursor.col != wp->w_ru_cursor.col
             || wp->w_virtcol != wp->w_ru_virtcol
             || wp->w_cursor.coladd != wp->w_ru_cursor.coladd
             || wp->w_topline != wp->w_ru_topline
             || wp->w_buffer->b_ml.ml_line_count != wp->w_ru_line_count
             || wp->w_topfill != wp->w_ru_topfill
             || empty_line != wp->w_ru_empty) {
    cursor_off();
// HERE
    int width;
    if (wp->w_status_height) {
      row = wp->w_winrow + wp->w_height;
      fillchar = fillchar_status(&attr, wp == curwin);
      off = wp->w_wincol;
      width = wp->w_width;
    } else {
      row = Rows - 1;
      fillchar = ' ';
      attr = 0;
      width = Columns;
      off = 0;
    }
```