Use iterator for STL std::map element access.

Use iterator for STL std::map element access.

The at() function for std::map was added for libstdc++ release 4.1, and
it is not part of the official STL.

Also some developers are still using libstdc++ 4.0, so using at() will
not compile.

Instead, use an iterator for std::map element access, which is the
standard (albeit annoying) practice.