hw/block/nvme: fix out-of-bounds read in nvme_subsys_ctrl

nvme_subsys_ctrl() is used in contexts where the given controller
identifier is from an untrusted source. Like its friends nvme_ns() and
nvme_subsys_ns(), nvme_subsys_ctrl() should just return NULL if an
invalid identifier is given.

Fixes: 645ce1a ("hw/block/nvme: support namespace attachment command")
Cc: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Minwoo Im <minwoo.im.dev@gmail.com>

hw/block/nvme-subsys.h

@@ -36,7 +36,7 @@ int nvme_subsys_register_ctrl(NvmeCtrl *n, Error **errp);
 static inline NvmeCtrl *nvme_subsys_ctrl(NvmeSubsystem *subsys,
         uint32_t cntlid)
 {
-    if (!subsys) {
+    if (!subsys || cntlid >= NVME_SUBSYS_MAX_CTRLS) {
         return NULL;
     }