BUG/MEDIUM: pools: fix the minimum allocation size

Fred reported a random crash related to the pools. This was introduced
by commit e18db9e98 ("MEDIUM: pools: implement a thread-local cache for
pool entries") because the minimum pool item size should have been
increased to 32 bytes to accommodate the 2 double-linked lists.

No backport is needed.

src/memory.c

@@ -62,10 +62,12 @@ struct pool_head *create_pool(char *name, unsigned int size, unsigned int flags)
 	 * ease merging of entries. Note that the rounding is a power of two.
 	 * This extra (void *) is not accounted for in the size computation
 	 * so that the visible parts outside are not affected.
+	 *
+	 * Note: for the LRU cache, we need to store 2 doubly-linked lists.
 	 */
 
 	if (!(flags & MEM_F_EXACT)) {
-		align = 16;
+		align = 4 * sizeof(void *); // 2 lists = 4 pointers min
 		size  = ((size + POOL_EXTRA + align - 1) & -align) - POOL_EXTRA;
 	}