mgmt: smp: Fix NULL pointer dereferences in UDP transport.

Guard call to k_thread_abort() to ensure it's not called with an
uninitialized thread and fix improper use of inet_pton() with
INADDR_ANY passed in place of a dotted-decimal string. Both of these
would otherwise lead to NULL pointer dereferences.

Signed-off-by: Berend Ozceri <berend@recogni.com>