Always check the return value of `repo_read_object_file()`

Always check the return value of `repo_read_object_file()`

There are a couple of places in Git's source code where the return value
is not checked. As a consequence, they are susceptible to segmentation
faults.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>