Don't crash in get_method() if self.mro is None -- might be because of a previous error. Fixes #1327.

@JukkaL: This is a band-aid; it's possible that a better solution
exists, e.g. marking such errors fatal, or constructing a better dummy
object.  But this prevents the immediate crash at hand, so we can move
on to more important issues.

Author: Guido van Rossum

mypy/nodes.py

@@ -1590,6 +1590,8 @@ def get_var_or_setter(self, name: str) -> SymbolNode:
         return self.get_var(name)
 
     def get_method(self, name: str) -> FuncBase:
+        if self.mro is None:  # Might be because of a previous error.
+            return None
         for cls in self.mro:
             if name in cls.names:
                 node = cls.names[name].node