dahdi_dynamic_eth: Prevent crash is packet arrives before span is fully configured.

It was possible after a dynamic ethernet span was created for a packet to come
in before the dahdi_span was fully initialized. The result would be a NULL
pointer dereference. Now just discard any packets that might come in during
this time window.

Internal-Issue-ID: DAHLIN-280
Reported-by: Pavel Selivanov
Signed-off-by: Shaun Ruffell <sruffell@digium.com>

Origin: http://svnview.digium.com/svn/dahdi?view=rev&rev=10587

git-svn-id: http://svn.asterisk.org/svn/dahdi/linux/branches/2.6@10591 a0bf4364-ded3-4de4-8d8a-66a801d63aff

drivers/dahdi/dahdi_dynamic_eth.c

@@ -71,6 +71,8 @@ static struct dahdi_span *ztdeth_getspan(unsigned char *addr, unsigned short sub
 	if (z)
 		span = z->span;
 	spin_unlock_irqrestore(&zlock, flags);
+	if (!span || !test_bit(DAHDI_FLAGBIT_REGISTERED, &span->flags))
+		return NULL;
 	return span;
 }