Tentative fix for crash in --quick mode (#3233)

Hopefully fixes #3214.

Author: gvanrossum

mypy/subtypes.py

@@ -135,11 +135,14 @@ def visit_instance(self, left: Instance) -> bool:
         if isinstance(right, TupleType) and right.fallback.type.is_enum:
             return is_subtype(left, right.fallback)
         if isinstance(right, Instance):
-            for base in left.type.mro:
-                if base._promote and is_subtype(
-                        base._promote, self.right, self.check_type_parameter,
-                        ignore_pos_arg_names=self.ignore_pos_arg_names):
-                    return True
+            # NOTO: left.type.mro may be None in quick mode if there
+            # was an error somewhere.
+            if left.type.mro is not None:
+                for base in left.type.mro:
+                    if base._promote and is_subtype(
+                            base._promote, self.right, self.check_type_parameter,
+                            ignore_pos_arg_names=self.ignore_pos_arg_names):
+                        return True
             rname = right.type.fullname()
             if not left.type.has_base(rname) and rname != 'builtins.object':
                 return False