get_header() always returns a non-NULL value, so checking

for NULL is certainly wrong and usually disables the
checks that we want to make instead.

This commit fixes a number of the above bugs where the result
of get_header() is immediately checked for NULL.

This is certainly a candidate for merging into 1.4




git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@46026 65c4cc65-6c06-0410-ace0-fbb531ad65f3