submodule: check for NULL return of get_submodule_ref_store()

submodule: check for NULL return of get_submodule_ref_store()

If we can't find a ref store for a submodule then assume the latter
is not initialized (or was removed).  Print a status line accordingly
instead of causing a segmentation fault by passing NULL as the first
parameter of refs_head_ref().

Reported-by: Jeremy Feusi <jeremy@feusi.co>
Reviewed-by: Stefan Beller <sbeller@google.com>
Initial-Test-By: Stefan Beller <sbeller@google.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>