remove almost all of the checks of the result from ast_strdupa() or alloca().

remove almost all of the checks of the result from ast_strdupa() or alloca().
As it turns out, all of these checks were useless, because alloca will never
return NULL.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@26451 65c4cc65-6c06-0410-ace0-fbb531ad65f3