WP_Press_This::_limit_string() needs to check if mb_strlen() is available

[ocean90]

trunk/src/wp-admin/includes/class-wp-press-this.php@32106#L329​

WP_Press_This::_limit_string() uses mb_strlen() but mbstring can be disabled. Therefore we need a check if the function exists.
mb_substr() is not affected because we provide a compat version.

Related: #30633

[SergeyBiryukov]

If we need a compat function, 31951.patch​ (from #29573) is similar to our current mb_substr() compat implementation.

Otherwise, just need a function_exists() check.

[azaozz]

31951.1.patch is the function_exists() check. The result is stored in a static to reduce calls to it.

I'm 50/50 on which to use. 31951.patch would be better for the long run, not sure if we should be introducing it right before RC.

[azaozz]

In 32114:

Add mb_strlen() compatibility function. Works the same way as the existing mb_substr() compatibility function.
Props SergeyBiryukov. Fixes #31951.

[SergeyBiryukov]

In 32115:

Fix a copy/paste error in a comment.

see #31951.