lib: Make scsi_free_scsi_task() accept a NULL task pointer

Several cleanup paths can be simplified if it is allowed to pass a
NULL pointer to scsi_free_scsi_task(). Hence modify this function
such that it returns immediately if passed a NULL task pointer.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

lib/scsi-lowlevel.c

@@ -66,6 +66,9 @@ scsi_free_scsi_task(struct scsi_task *task)
 {
 	struct scsi_allocated_memory *mem;
 
+	if (!task)
+		return;
+
 	while ((mem = task->mem)) {
 		   SLIST_REMOVE(&task->mem, mem);
 		   free(mem);