Ensure we are on a multisite before using switch_to_blog

[dkotter]

Description of the Change

As detailed in #773, if you use Distributor on a multisite and then convert some of those sites into single install sites, you can run into PHP fatal errors. The issue here is around our usage of switch_to_blog. We had checks in place to ensure we only used that if the proper Distributor meta was in place but in the above scenario, you can have that meta but actually not be on a multisite.

This PR fixes that by adding is_multisite() checks to any function that uses switch_to_blog after checking for the existence of Distributor meta. This should prevent fatal errors from occurring in the above detailed scenario.

Alternate Designs

None

Benefits

No PHP fatal errors will happen when using Distributor on sites that used to be part of a multisite

Possible Drawbacks

None

Verification Process

1. Create a WordPress multisite with two sites
2. Distribute some content from site A to site B
3. Convert site B to a stand-alone site
4. Edit one of the items that was distributed and see that no errors occur

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

#773

Changelog Entry

Fixed - ensure we are on a multisite before using switch_to_blog

[helen]

LGTM