Don't crash when passing a NULL message to __astman_get_header.
Before this commit, __astman_get_header would blindly dereference the passed in
'struct message *' to traverse the header list. There are cases, however, such
as '*CLI> sip qualify peer foo' where the message pointer is NULL, so we need
to check for that.
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Merged revisions 373131 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373132 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373133 from http://svn.asterisk.org/svn/asterisk/branches/11
git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373134
65c4cc65-6c06-0410-ace0-
fbb531ad65f3