fix(select): prevent change detection of destroyed option

[yggg]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

Closes #1210.

As turned out this issue related to reactive forms (angular/angular#27803).
writeValue called even if ng-select bound to [formControl] directive already destroyed.

[codecov]

Codecov Report

Merging #1329 into master will increase coverage by 0.03%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #1329      +/-   ##
==========================================
+ Coverage   80.65%   80.68%   +0.03%     
==========================================
  Files         232      232              
  Lines        7036     7032       -4     
  Branches      593      593              
==========================================
- Hits         5675     5674       -1     
+ Misses       1187     1184       -3     
  Partials      174      174

Impacted Files	Coverage Δ
...mework/theme/components/select/select.component.ts	89.72% <83.33%> (+0.95%)	⬆️
...mework/theme/components/select/option.component.ts	94.59% <83.33%> (+2.28%)	⬆️