[reSIProcate] expires Contact parameter can be parsed as negative if too large

[Byron Campen]

	In the scalar02 torture-tests case, an over-large Contact expires  
param will be parsed as negative, and can be re-emitted.

*snip*
In case scalar02:
Original text:
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/TCP host129.example.com;branch=z9hG4bK342sdfoi3
To: <sip:user at example.com>
From: <sip:user at example.com>;tag=239232jh3
CSeq: 36893488147419103232 REGISTER
Call-ID: scalar02.23o0pd9vanlq3wnrlnewofjas9ui32
Max-Forwards: 300
Expires:  
100000000000000000000000000000000000000000000000000000000000000000000000 
00000000000000000000000000000
Contact: <sip:user at host129.example.com>
   ;expires=280297596632815
Content-Length: 0


Encoded form:
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/TCP host129.example.com;branch=z9hG4bK342sdfoi3
Max-Forwards: 300
Contact: <sip:user at host129.example.com>;expires=-559038737
To: <sip:user at example.com>
From: <sip:user at example.com>;tag=239232jh3
Call-ID: scalar02.23o0pd9vanlq3wnrlnewofjas9ui32
CSeq: 0 REGISTER
Expires: 0
Content-Length: 0

*snip*

Best regards,
Byron Campen

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