glusterfs: fix heap-buffer-overflow on startup

==4418==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x613000000190
at pc 0x7f028cd2341c bp 0x7ffd9c5ba7e0 sp 0x7ffd9c5ba7d8
WRITE of size 8 at 0x613000000190 thread T0
    f0 in mem_get_pool_list ~/libglusterfs/src/mem-pool.c:786

0x613000000190 is located 0 bytes after 336-byte region [0x613000000040,0x613000000190)
allocated by thread T0 here:
    f0 in malloc (/usr/lib64/libasan.so.8+0xdc04f) (BuildId: 44194dcf14c212b57346030492309d59d5379ae1)
    f1 in __gf_default_malloc glusterfs/mem-pool.h:112
    f2 in mem_get_pool_list ~/libglusterfs/src/mem-pool.c:778

``NPOOLS-1`` is just wrong. ``per_thread_pool_list_t`` does not include one free
``per_thread_pool_t``.

Fixes: gluster#4192
Fixes: v11dev-211-g1cfff6e6ec ("Use flexible array members (gluster#3411)")
Signed-off-by: Jan Engelhardt <jengelh@inai.de>

libglusterfs/src/mem-pool.c

@@ -589,8 +589,7 @@ mem_pools_preinit(void)
     }
 
     pool_list_size = sizeof(per_thread_pool_list_t) +
-                     sizeof(per_thread_pool_t) * (NPOOLS - 1);
-
+                     sizeof(per_thread_pool_t) * NPOOLS;
     init_done = GF_MEMPOOL_INIT_EARLY;
 }