Unlike ast_strdup(), ast_strdupa() does not take a NULL pointer as argument,

Unlike ast_strdup(), ast_strdupa() does not take a NULL pointer as argument,
so fix the places where this might happen.

This is also a fix that ought to go into 1.4

[The difference between the two functions is a bit confusing,
and in asterisk i believe all string handling functions
should be able to handl a NULL string as argument,
but changing the API in trunk and not in 1.4 would make
backporting harder.]

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@46045 65c4cc65-6c06-0410-ace0-fbb531ad65f3