list-objects.c: handle unexpected non-blob entries

list-objects.c: handle unexpected non-blob entries

Fix one of the cases described in the previous commit where a tree-entry
that is promised to a blob is in fact a non-blob.

When 'lookup_blob()' returns NULL, it is because Git has cached the
requested object as a non-blob. In this case, prevent a SIGSEGV by
'die()'-ing immediately before attempting to dereference the result.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>