ssh-agent: Fix leak in remove_by_public_key()

gck_attributes_find_string() returns a newly allocated string, so it
must be g_free'ed after use.

https://bugzilla.gnome.org/show_bug.cgi?id=738508
ssh-agent: Fix leak in search_keys_like_attributes()

gck_enumerator_next() return value must be unref'fed as it's transfer
full.

https://bugzilla.gnome.org/show_bug.cgi?id=738508