Prevent fatal errors for sub shortcodes

[derekashauer]

All Submissions:

• Have you followed the Contributing guidelines?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Resolves #22

How to test the changes in this Pull Request:

1. Use any sub shortcodes out of context (without main shortcode). Example:

[pmpro_testimonials_display_custom]
[pmpro_testimonial_content]
[pmpro_testimonial_rating]

2. Errors should no longer appear

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

Prevent sub shortcodes from showing fatal error when not used in proper context