Bluetooth: btintel: Validate length before parsing diagnostics TLV

Bluetooth: btintel: Validate length before parsing diagnostics TLV

btintel_diagnostics() accesses tlv->val[0] without first validating
that the diagnostics VSE is long enough to contain that field, so
may cause reading data beyond the received frame.

Fix by validating the length before access.

Fixes: af395330abed ("Bluetooth: btintel: Add Intel devcoredump support")
Signed-off-by: Zijun Hu <zijun.hu@oss.qualcomm.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>