commit-slab: sizeof() the right type in xrealloc

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: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Thomas Rast <tr@thomasrast.ch>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>