chan_sip: Update the via header when relaying SMS MESSAGE

Prior to this change, certain conditions for sending the message would
result in an address of '(null)' being used in the via header of the
SIP message because a NULl value of pvt->ourip was used when initially
generating the via header. This is fixed by adding a call to build_via
when the address is set before sending the message.

(closes issue ASTERISK-21148)
Reported by: Zhi Cheng
Patches:
	700-sip_msg_send_via_fix.patch uploaded by Zhi Cheng (license 6475)


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@382739 f38db490-d61c-443f-a65b-d21fe96a405b