fix warning from -Wsign-compare

lib/scsi-lowlevel.c

@@ -1082,7 +1082,7 @@ scsi_get_task_private_ptr(struct scsi_task *task)
 
 struct scsi_data_buffer {
        struct scsi_data_buffer *next;
-       int len;
+       uint32_t len;
        unsigned char *data;
 };
 
@@ -1091,6 +1091,9 @@ scsi_task_add_data_in_buffer(struct scsi_task *task, int len, unsigned char *buf
 {
 	struct scsi_data_buffer *data_buf;
 
+	if (len < 0) {
+		return -1;
+	}
 	data_buf = scsi_malloc(task, sizeof(struct scsi_data_buffer));
 	if (data_buf == NULL) {
 		return -1;