fix a flaw found while experimenting with structure alignment and padding; low-fence...

fix a flaw found while experimenting with structure alignment and padding; low-fence checking would not work properly on 64-bit platforms, because the compiler was putting 4 bytes of padding between the fence field and the allocation memory block

added a very obvious runtime warning if this condition reoccurs, so the developer who broke it can be chastised into fixing it :-)

git-svn-id: http://svn.digium.com/svn/asterisk/branches/1.4@129966 614ede4d-c843-0410-af14-a771ab80d22e