hdlc: fix null-deref on allocation failure

If alloc_netdev() failed and return NULL, then the next instruction
would dereference it. Found by Coverity.

Compile tested only. Not sure if anyone still uses this driver
(or the whole WAN subsystem).

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>