Expression parser: Optimize another concatenation

Expression parser: Optimize another concatenation
case by using iteration instead of recursion.

We have a relatively small recursion limit of
about 10 operations. This is a compilation
limit (a define). It can be hit if many expr
vars or function calls are concatenated.

The new optimization is very similar to the
existing one, which optimizes consecutive
concatenations in node2 of the tree. The new
one optimizes consecutive concatenations in
node 1.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1657685 13f79535-47bb-0310-9956-ffa450edef68