mISDN: Fix null pointer dereference at mISDN_FsmNew

If mISDN_FsmNew() fails to allocate memory for jumpmatrix
then null pointer dereference will occur on any write to
jumpmatrix.

The patch adds check on successful allocation and
corresponding error handling.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Anton Vasilyev <vasilyev@ispras.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>