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: 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>

commit-slab.h

@@ -91,7 +91,7 @@ static MAYBE_UNUSED elemtype *slabname## _at(struct slabname *s,	\
 	if (s->slab_count <= nth_slab) {				\
 		int i;							\
 		s->slab = xrealloc(s->slab,				\
-				   (nth_slab + 1) * sizeof(s->slab));	\
+				   (nth_slab + 1) * sizeof(*s->slab));	\
 		stat_ ##slabname## realloc++;				\
 		for (i = s->slab_count; i <= nth_slab; i++)		\
 			s->slab[i] = NULL;				\