Initialize machine to None

This follows the logic in the _v2 version of this function.

Resolves #4588

CIME/XML/machines.py

@@ -263,6 +263,7 @@ def _probe_machine_name_one_guess_v3(self, nametomatch):
 
         children = [y for x in nodes for y in self.get_children(root=x)]
 
+        machine = None
         for child in children:
             machtocheck = self.get(child, "MACH")
             regex_str = self.text(child)