check_aliased_update(): check that dst_name is non-NULL

If there is an error in resolve_ref_unsafe(), it returns NULL. We check
for this case, but not until after calling strip_namespace(). Instead,
call strip_namespace() *after* the NULL check.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>