net: ppp: Fix NULL pointer dereference in FSM module

ppp_send_pkt() function can be called with NULL fsm parameter (when
PPP_PROTOCOL_REJ packet was sent), howerver this was not taken into
consideration when ppp_context was retrieved. In result, this could lead
to NULL pointer dereference an crash.

Fix this, by moving the ppp_context extraction directly where it's
actually used (PPP_CODE_REJ packet type handling). In such case, fsm
point should not be NULL.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>