write-tree: Avoid leak when index refers to an invalid object

write-tree: Avoid leak when index refers to an invalid object

Noticed by valgrind during test t0000.35 “writing this tree without
--missing-ok”.

Even in the cherry-pick foo..bar code path, such an error is the
end of the line.  But maybe some day an interactive porcelain will
want to link to libgit, making this matter.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>