#47 stun tport not initialized if NUTAG_URL given

[Kai Vehmanen]

If NUTAG_ULR() is given to nua_create(), the stack will
not try to initialize STUN contact for SIP signaling.

This can be reproduced with sofsip_cli by setting a
valid STUN server address to SOFSIP_STUN_SERVER
environment variable, and the starting with and without
'--contact=sip-uri' parameter.

[Kai Vehmanen]

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Verified with 1.12.3, problem persists.