When allocating the slab, the code accidentally computed the array
size from s->slab (an elemtype**).  The slab is an array of elemtype*,
however, so we should take the size of *s->slab.
Noticed-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NThomas Rast <tr@thomasrast.ch>
Reviewed-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>