efidp_is_multiinstance(): check for efidp_next_node() == 0 in our iter

efidp_is_multiinstance(): check for efidp_next_node() == 0 in our iter

GCC doesn't seem to want to believe that the NULL and length tests in
in efidp_is_multiinstance()->efidp_next_node()->efidp_node_size()
actually catches things, Possibly because it wouldn't be acted on until
the next iteration through the loop.   So check that case explicitly in
efidp_is_multiinstance().  Also forcibly set 'next' to NULL, to ensure
efidp_next_node() will catch it.

Signed-off-by: Peter Jones <pjones@redhat.com>