Fixing up success message if adding self to institution (Bug #1358481)

When adding user to institution on admin/users/edit.php page and the
user you are editing is the same user you are logged in as it cause an
array missing index error.

Change-Id: I4afef6f8998598f77d4f135afd380f725fc310fe
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Merge "Fixing up success message if adding self to institution (Bug #1358481)"