Member Export outputs duplicates when using pmpro_set_max_user_per_export_loop filter

[MaximilianoRicoTabo]

• Ensure the filter doesn't lead to a fatal error by ensuring the returned value is valid or set the default
• Fix array_slice start and end params to calculations.

### ERROR SCREENSHOT

### FIXED SCREENSHOTS

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Changes proposed in this Pull Request:

Added intval to the filter result of $max_users_per_loop and set the default if it's 0. Any non integer value or 0 would through a fatal error.

Also, better calculate iteration start and end to avoid duplicates while iterate.

Resolves #3236 .

How to test the changes in this Pull Request:

Follow steps described in #3236

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.