Merge pull request #4596 from billsacks/fix_unbound_machine

Initialize machine to None

This follows the logic in the _v2 version of this function, as well as the documentation in "_probe_machine_name_one_guess" ("Returns None if no match is found").

Test suite: None (just manually tested the issue documented in #4588 ).
Test baseline: none
Test namelist changes: none
Test status: bit for bit

Fixes #4588

User interface changes?: N

Update gh-pages html (Y/N)?: N

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)