Fix potential reentrancy problems in chan_sip.

Asterisk v1.8 and later was not as vulnerable to this issue.

* Made find_call() lock each private as it processes the found dialogs.
(Primary cause of ABE-2876)

* Made the other functions that traverse the dialogs container lock each
private as it examines them.

* Fix race condition in sip_call() if the thread that sent the INVITE is
held up long enough for a response to be processed.  The p->initid for the
INVITE retransmission could be added after it was canceled by the response
processing.

* Made __sip_destroy() clean up resource pointers after freeing.  This is
primarily defensive in case someone has a stale private pointer.

* Removed redundant memset() in reqprep().  The call to init_req() already
does the memset() and is the first reference to req in reqprep().

* Removed useless set of req.method in transmit_invite().  The calls to
initreqprep() and reqprep() have to do this because they memset() the req.

JIRA ABE-2876

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Merged -r373423 from https://origsvn.digium.com/svn/asterisk/be/branches/C.3-bier
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Merged revisions 373424 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373466 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373469 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373471 65c4cc65-6c06-0410-ace0-fbb531ad65f3