NVMe: Protect against badly formatted CQEs

If a misbehaving device posts a CQE with a command id < depth but for
one that was never allocated, the command info will have a callback
function set to NULL and we don't want to try invoking that.
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>

drivers/block/nvme-core.c

@@ -243,8 +243,9 @@ static void *free_cmdid(struct nvme_queue *nvmeq, int cmdid,
 	void *ctx;
 	struct nvme_cmd_info *info = nvme_cmd_info(nvmeq);
 
-	if (cmdid >= nvmeq->q_depth) {
-		*fn = special_completion;
+	if (cmdid >= nvmeq->q_depth || !info[cmdid].fn) {
+		if (fn)
+			*fn = special_completion;
 		return CMD_CTX_INVALID;
 	}
 	if (fn)