isdn_ppp: Add checks for allocation failure in isdn_ppp_open()

[ Upstream commit 0baa57d8dc32db78369d8b5176ef56c5e2e18ab3 ]

Compile-tested only.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>