Code cleanup and bugfix in chan_sip outboundproxy parsing.

The bug was clearing the global outboundproxy when a peer-specific
outboundproxy was bad. The cleanup reduces duplicate code.

Review: https://reviewboard.asterisk.org/r/2034/
Reviewed by: Mark Michelson
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Merged revisions 370131 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 370132 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370133 65c4cc65-6c06-0410-ace0-fbb531ad65f3