BUG/MEDIUM: vars: make sure the scope is always valid when accessing vars

BUG/MEDIUM: vars: make sure the scope is always valid when accessing vars

Patrick Hemmer reported that a simple tcp rule involving a variable like
this is enough to crash haproxy :

    frontend foo
        bind :8001
        tcp-request session set-var(txn.foo) src

The tests on the variables scopes is not strict enough, it needs to always
verify if the stream is valid when accessing a req/res/txn variable. This
patch does this by adding a new get_vars() function which does the job
instead of open-coding all the lookups everywhere.

It must be backported to all versions supporting set-var and
"tcp-request session" so at least 1.9 and 1.8.