various code simplifications to reduce nesting depth,

various code simplifications to reduce nesting depth,
minor optimizations to avoid extra calls of strlen(),
and some variable localization.

One feature worth backporting is the move of ast_variables_destroy()
to a different place in handle_uri() to avoid leaking memory
in case a uri is not found.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@45463 65c4cc65-6c06-0410-ace0-fbb531ad65f3