reject partitions with negative offset

I had a KDL when trying to read an audio CD which apparently uses this
as a copy protection scheme.
I don't know if this is the right place to do this, the KDL would happen
further down when the intel partitionning system or bfs would try to
read data from the disk at offset -2048.

src/system/kernel/disk_device_manager/KDiskDeviceManager.cpp

@@ -1373,6 +1373,10 @@ KDiskDeviceManager::_ScanPartition(KPartition* partition,
 		return B_OK;
 	}
 
+	// This happens with some copy protected CDs. Just ignore the partition...
+	if (partition->Offset() < 0)
+		return B_BAD_DATA;
+
 	DBG(
 		KPath partitionPath;
 		partition->GetPath(&partitionPath);