Fix DTE accessing omit v_Entity.sCTP_Assoc_ID

It may happen that as_Unexpected_SCTP_Events() is triggered when the association is still not iset/present (omit), eg. upon connection timeout.

18:15:12.043014 20 M3UA_Emulation.ttcn:581 Timeout T_Timer: 2 s
18:15:12.043056 20 M3UA_Emulation.ttcn:582 ----------------------------------------------
18:15:12.043074 20 M3UA_Emulation.ttcn:583 No response received to t_ASP_SCTP_ConnectFrom
18:15:12.043100 20 M3UA_Emulation.ttcn:584 ----------------------------------------------
18:15:12.043157 20 M3UA_Emulation.ttcn:585 setverdict(fail): none -> fail
18:15:12.043178 20 M3UA_Emulation.ttcn:537 Start timer T_Assoc_restart: 60 s
18:15:12.043226 20 M3UA_Emulation.ttcn:542 Start timer T_ASPUP_resend: 2 s
18:15:12.043235 20 M3UA_Emulation.ttcn:543 Start timer T_ASPAC_resend: 2 s
18:15:12.043322 20 M3UA_Emulation.ttcn:597 Matching on port MTP3_SP_PORT succeeded:  matched
18:15:12.043355 20 M3UA_Emulation.ttcn:597 Receive operation on port MTP3_SP_PORT succeeded, message from SGSN_Test_0-SCCP(18): @MTP3asp_Types.ASP_MTP3_TRANSFERr
eq : { sio := { ni := '10'B, prio := '00'B, si := '0011'B }, opc := 195, dpc := 188, sls := 0, data := '090003070B0443BC008E0443C3008E110009000D00000200044001400003000180'O } id 1
18:15:12.043388 20 M3UA_Emulation.ttcn:597 Message with id 1 was extracted from the queue of MTP3_SP_PORT.
18:15:12.043608 20 M3UA_Emulation.ttcn:668 Matching on port SCTP_PORT SCTP_CANT_STR_ASSOC (4) with SCTP_COMM_UP (0) unmatched: First message in the queue does not match the template:
18:15:12.043688 20 M3UA_Emulation.ttcn:711 Matching on port SCTP_PORT failed: Type of the first message in the queue is not @SCTPasp_Types.ASP_SCTP.
18:15:12.043731 20 M3UA_Emulation.ttcn:781 Matching on port SCTP_PORT SCTP_CANT_STR_ASSOC (4) with SCTP_COMM_LOST (1) unmatched: First message in the queue does not match the template:
18:15:12.043743 20 M3UA_Emulation.ttcn:802 Matching on port SCTP_PORT SCTP_CANT_STR_ASSOC (4) with SCTP_SHUTDOWN_COMP (3) unmatched: First message in the queue does not match the template:
18:15:12.043774 20 M3UA_Emulation.ttcn:822 Matching on port SCTP_PORT failed: Type of the first message in the queue is not @SCTPasp_Types.ASP_SCTP_SHUTDOWN_EVENT.
18:15:12.043800 20 M3UA_Emulation.ttcn:842 Matching on port SCTP_PORT failed: Type of the first message in the queue is not @SCTPasp_Types.ASP_SCTP_Connected.
18:15:12.043816 20 M3UA_Emulation.ttcn:847 Matching on port SCTP_PORT SCTP_CANT_STR_ASSOC (4) with SCTP_RESTART (2) unmatched: First message in the queue does not match the template:
18:15:12.043834 20 M3UA_Emulation.ttcn:863 Matching on port SCTP_PORT failed: Type of the first message in the queue is not @SCTPasp_Types.ASP_SCTP_SEND_FAILED.
18:15:12.043867 20 M3UA_Emulation.ttcn:875 Matching on port SCTP_PORT succeeded:  matched
18:15:12.043889 20 M3UA_Emulation.ttcn:875 Receive operation on port SCTP_PORT succeeded, message from system(): @SCTPasp_Types.ASP_SCTP_ASSOC_CHANGE : { client_id := 8, sac_state := SCTP_CANT_STR_ASSOC (4) } id 1
18:15:12.043914 20 M3UA_Emulation.ttcn:875 Message with id 1 was extracted from the queue of SCTP_PORT.
18:15:12.043925 20 M3UA_Emulation.ttcn:668 Matching on port SCTP_PORT SCTP_COMM_LOST (1) with SCTP_COMM_UP (0) unmatched: First message in the queue does not match the template:
18:15:12.043934 20 M3UA_Emulation.ttcn:711 Matching on port SCTP_PORT failed: Type of the first message in the queue is not @SCTPasp_Types.ASP_SCTP.
18:15:12.043943 20 M3UA_Emulation.ttcn:781 Matching on port SCTP_PORT succeeded:  matched
18:15:12.043950 20 M3UA_Emulation.ttcn:781 Receive operation on port SCTP_PORT succeeded, message from system(): @SCTPasp_Types.ASP_SCTP_ASSOC_CHANGE : { client_id := 8, sac_state := SCTP_COMM_LOST (1) } id 2
18:15:12.043965 20 M3UA_Emulation.ttcn:781 Message with id 2 was extracted from the queue of SCTP_PORT.
18:15:12.043999 20 M3UA_Emulation.ttcn:980 Dynamic test case error: The left operand of comparison is an unbound optional value.