[POWERPC] mpc5200: Fix null dereference if bestcomm fails to initialize

If the bestcomm initialization fails, calls to the task allocate
function should fail gracefully instead of oopsing with a NULL deref.
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/sysdev/bestcomm/bestcomm.c

@@ -52,6 +52,10 @@ bcom_task_alloc(int bd_count, int bd_size, int priv_size)
 	int i, tasknum = -1;
 	struct bcom_task *tsk;
 
+	/* Don't try to do anything if bestcomm init failed */
+	if (!bcom_eng)
+		return NULL;
+
 	/* Get and reserve a task num */
 	spin_lock(&bcom_eng->lock);