lib/sg_pool.c: remove unnecessary null check when freeing object

mempool_destroy(NULL) and kmem_cache_destroy(NULL) are legal

Link: http://lkml.kernel.org/r/1533054107-35657-1-git-send-email-zhongjiang@huawei.com
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>