traverse_elements: early return from recursion
traverse_elements: early return from recursion

Makes get_element_by_qname more predictable. Makes tests pass.

bkircher/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,otris/ews-cpp,m-Schlitzer/ews-cpp,m-Schlitzer/ews-cpp,m-Schlitzer/ews-cpp,otris/ews-cpp,otris/ews-cpp,bkircher/ews-cpp