Replace wp_guess_url() with bp_get_requested_url().

wp_guess_url() doesn't work well with subdirectory installs and anyway,
bp_get_requested_url() is superior!

Fixes #5737.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8680 cdf35c40-ae34-48e0-9cc9-0c9da1808c22