Catch errors when writing an index that contains invalid objects.

If git-write-index is called without --missing-ok, it reports invalid
objects that it finds in the index. But without this patch it dies
right away or may run into an infinite loop.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <junkio@cox.net>