Fix crash if "_" is in builtins (python#11811)

allows the definition of "_" as function in builtins (e.g via typeshed).

fixes python#9802

mypy/semanal.py

@@ -4296,7 +4296,7 @@ def lookup(self, name: str, ctx: Context,
             assert isinstance(b.node, MypyFile)
             table = b.node.names
             if name in table:
-                if name[0] == "_" and name[1] != "_":
+                if len(name) > 1 and name[0] == "_" and name[1] != "_":
                     if not suppress_errors:
                         self.name_not_defined(name, ctx)
                     return None