git-fast-import possible memory corruption problem

git-fast-import possible memory corruption problem

Internal "allocate in bulk, we will never free this memory anyway"
allocator used in fast-import had a logic to round up the size of the
requested memory block in a wrong place (it computed if the available
space is enough to fit the request first, and then carved a chunk of
memory by size rounded up to the alignment, which could go beyond the
actually available space).

Signed-off-by: Junio C Hamano <gitster@pobox.com>