[iOS] BugFix, slide.index is reset to zero when the view is out of wi…

[mahaiyannn]

…ndow. It should be kept to the last show index.

First of all, thank you for your contribution!

Please makes sure that these checkboxes are checked before submitting your PR, thank you!

CheckList:

if isBugFix :

• Make sure that you add at least one unit test for the bug which you had fixed.

elif isNewFeature :

• Update API docs for the component.
• Update/Add demo to demonstrate new feature.
• Update test scripts for the feature.
• Add unit tests for the feature.

[weex-bot]

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📖	No android file has been changed.
📖	android build verification finished.

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[wqyfavor]

ok