New pack structures are currently allocated in 2 different places
and all members have to be initialized explicitly. This is prone
to errors leading to segmentation faults as found by Teemu Likonen.
Let's have a common place where this structure is allocated, and have
all members explicitly initialized to zero.
Signed-off-by: Nicolas Pitre <nico@cam.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>