bundle: fix wrong check of read_header()'s return value & add tests

If read_header() fails, it returns <0, not 0. Further, an open(/dev/null)
was not checked for errors.

Also, this adds two tests to make sure that the bundle file looks
correct, by checking if it has the header has the expected form, and that
the pack contains the right amount of objects.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>