branch: don't assume the merge filter ref exists

branch: don't assume the merge filter ref exists

print_ref_list looks up the merge_filter_ref and assumes that a valid
pointer is returned. When the object doesn't exist, it tries to
dereference a NULL pointer. This can be the case when git branch
--merged is given an argument that isn't a valid commit name.

Check whether the lookup returns a NULL pointer and die with an error
if it does. Add a test, while we're at it.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>