_bp_strip_spans_from_title() causing fatal errors in 2.2.2

[DJPaul]

r9732 (#6107) introduced this fatal error:

Fatal error: Cannot redeclare _bp_strip_spans_from_title() (previously declared in /wp-content/plugins/buddypress/bp-core/bp-core-filters.php:552) in /wp-content/plugins/buddypress/bp-core/bp-core-filters.php on line 552

This is because _bp_strip_spans_from_title is nested inside bp_modify_page_title. If a site calls any of WP's title functions more than once (for example, wp_title or get_the_title), it triggers the fatal error. You can reproduce this easily by adding <?php wp_title(); ?> into the bottom of the active theme's footer.php, for example.

Thanks to @danbp for raising awareness in Slack, via @imath. First reported on the forums:

• ​https://buddypress.org/support/topic/fatal-error-after-update/
• ​https://buddypress.org/support/topic/groups-function-not-working/

Relevant Slack logs of the investigation/discussion are at ​https://wordpress.slack.com/archives/buddypress/p1428739967001905

[DJPaul]

From limited investigation of one of the reported sites that have this problem, I was unable to find a plugin that was triggering this problem, but imath and I are pretty sure that we have identified the root cause, even if we can't point to some theme/plugin/widget that demonstrates the problem.

[djpaul]

In 9741:

Core: move _bp_strip_spans_from_title() out of bp_modify_page_title().

The nested function was causing PHP Fatal Errors ("cannot redeclare") when WP's title functions were invoked more than once on the same screen.
This was introduced in r9732.

Fixes #6369