Slideshow: Fix Javascript Error After Adding Block

[jeffersonrabb]

Changes proposed in this Pull Request:

Fixes a Javascript error that occurs when Slideshow block is first added.

Testing instructions:

• On local instance install Gutenberg plugin (v.5.3 or higher). The issue won't occur without the plugin.
• Check out master, then run yarn && yarn build-extensions
• Create a new Post, add Slideshow block, choose an image and create a gallery.
• The block should fail with a JS error, as described in New Slideshow Block Errors Out In Gutenberg 5.3 #11702
• Check out fix/slideshow-realindex-error and repeat the steps above. This time there should be no error.

Fixes #11702

[jetpackbot]

	Warnings
⚠️	The PR is missing at least one [Status] label. Suggestions: [Status] In Progress, [Status] Needs Review
⚠️	"Proposed changelog entry" is missing for this PR. Please include any meaningful changes

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

Generated by 🚫 dangerJS against 643a0dc

[sirreal]

Fixes the issue 👍

[sirreal]

Need to pull this in to Automattic/wp-calypso#31793 and release a fixed jetpack-blocks package.

[jeherve]

I can't seem to reproduce the issue right now, but it still works with this branch ¯_(ツ)_/¯

[lezama]

merged to Automattic/wp-calypso#31793 in Automattic/wp-calypso@5a438f0