Make sure the current administration screen ID is set before checking its value

In {{{BP_Members_Admin->remove_signups_from_user_query()}}}, to avoid a notice error, we need to check that {{{get_current_screen()->id}}} is set.

Props boonebgorges

Fixes #5843

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8984 cdf35c40-ae34-48e0-9cc9-0c9da1808c22