Don't crash when passing a NULL message to __astman_get_header.

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

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@373133 65c4cc65-6c06-0410-ace0-fbb531ad65f3