merge-ort: fix segmentation fault in read-only repositories
If the blob/tree objects cannot be written, we really need the merge
operations to fail, and not to continue (and then try to access the tree
object which is however still set to `NULL`).
Let's stop ignoring the return value of `write_object_file()` and
`write_tree()` and set `clean = -1` in the error case.
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>