fix out-of-bounds active_option crash

[hernanat]

fixes bug where active_option index becomes out of bounds when the
options list is dynamically updated via send_update, causing the
component to crash with KeyError or BadMapError when trying to
access .label on nil.

Variant 1: Crash in already_selected?

Steps to reproduce:

1. Open a LiveSelect component
2. Type a search query that returns multiple options
3. Use arrow keys to navigate to an option towards the end of the list
4. Select that option
5. Re-open or re-focus the same LiveSelect
6. Type a new search query that returns FEWER options than before
7. Use arrow keys to navigate down in this shorter list
8. Type additional characters to narrow the search even MORE (making the list even shorter)
9. Press Enter to select

Variant 2: Crash in new_current_text_after_selection

Steps to reproduce:

1. Open a LiveSelect component
2. Type a search query that returns multiple options
3. Use arrow keys to navigate to an option towards the end of the list
4. Do NOT press Enter yet - keep the option highlighted
5. Type additional characters to filter results
6. Press Enter to select

[hernanat]

@maxmarcon this one has been rearing its head for us for some time now (we are on 1.7.4, but we have sentries from ~ 10 months ago), finally had time to look into it ... hope this helps!

[apreifsteck]

I have also noticed this bug a few times and would love to see a fix!

[3zcurdia]

I'll love to see this fix into the next release, thanks @hernanat

[maxmarcon]

Thanks @hernanat ! will take a look soon

[maxmarcon]

Merged and released with version 1.7.5

[hernanat]

whoo! thanks!