read-cache: fix mem-pool allocation for multi-threaded index loading

44c7e1a7 (mem-pool: use more standard initialization and finalization,
2020-08-15) moved the allocation of the mem-pool structure to callers.
It also added an allocation to load_cache_entries_threaded(), but for an
unrelated mem-pool.  Fix that by allocating the correct one instead --
the one that is initialized two lines later.
Reported-by: NSandor Bodo-Merle <sbodomerle@gmail.com>
Signed-off-by: NRené Scharfe <l.s.r@web.de>
Reviewed-by: NElijah Newren <newren@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

read-cache.c

@@ -2101,7 +2101,7 @@ static unsigned long load_cache_entries_threaded(struct index_state *istate, con
 		nr = 0;
 		for (j = p->ieot_start; j < p->ieot_start + p->ieot_blocks; j++)
 			nr += p->ieot->entries[j].nr;
-		istate->ce_mem_pool = xmalloc(sizeof(*istate->ce_mem_pool));
+		p->ce_mem_pool = xmalloc(sizeof(*istate->ce_mem_pool));
 		if (istate->version == 4) {
 			mem_pool_init(p->ce_mem_pool,
 				estimate_cache_size_from_compressed(nr));