We currently call parse_commit and then assume we can
dereference the resulting "tree" struct field. If parsing
failed, however, that field is NULL and we end up
segfaulting.
Instead of a segfault, let's print an error message and die
a little more gracefully.
Note that this should never happen in practice, but may
happen in a corrupt repository.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>