switch_to_blog does not check if the blog really exists

[wpmuguru]

MU Trac Ticket: ​http://trac.mu.wordpress.org/ticket/1162

switch_to_blog function imo should check if the blog specified exists or not, if not return FALSE instead of TRUE.

[wpmuguru]

(In [13047]) add option to validate on switch_to_blog, see #12023

[wpmuguru]

To check if the blog really exists when using switch_to_blog in a plugin, add a second parameter true

$exists = switch_to_blog($blog_id, true);