Fix BaseXMLResponseParser._handle_map to not crash

key_name and val_name are set inside of the loop over
keyval_node pairs.  The crash is caused by key_name and
val_name being referenced from outside of the loop, where
they may be used before they have been set.

Fix this by setting key_name and val_name to None before
the loop, and set parsed[key_name] to val_name only if both
key_name and val_name have both been given values from the
loop.