isinstance with single argument causes crash

[JukkaL]

This crashes mypy:

isinstance(1)

Traceback:

Traceback (most recent call last):
  File "/Users/jukka/src/mypy/scripts/mypy", line 6, in <module>
    main(__file__)
  File "/Users/jukka/src/mypy/mypy/main.py", line 50, in main
    res = type_check_only(sources, bin_dir, options)
  File "/Users/jukka/src/mypy/mypy/main.py", line 97, in type_check_only
    options=options)
  File "/Users/jukka/src/mypy/mypy/build.py", line 192, in build
    graph = dispatch(sources, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 1761, in dispatch
    process_graph(graph, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2004, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/Users/jukka/src/mypy/mypy/build.py", line 2107, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/Users/jukka/src/mypy/mypy/build.py", line 1676, in type_check_first_pass
    self.type_checker.check_first_pass()
  File "/Users/jukka/src/mypy/mypy/checker.py", line 185, in check_first_pass
    self.accept(d)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 273, in accept
    stmt.accept(self)
  File "/Users/jukka/src/mypy/mypy/nodes.py", line 829, in accept
    return visitor.visit_expression_stmt(self)
  File "/Users/jukka/src/mypy/mypy/checker.py", line 1880, in visit_expression_stmt
    self.expr_checker.accept(s.expr, allow_none_return=True, always_allow_any=True)
  File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 2223, in accept
    typ = self.visit_call_expr(node, allow_none_return=True)
  File "/Users/jukka/src/mypy/mypy/checkexpr.py", line 196, in visit_call_expr
    for typ in mypy.checker.flatten(e.args[1]):
IndexError: list index out of range

[ilevkivskyi]

My fault, missed this when reviewing PR #3040 (fortunately this is easy to fix).

[JukkaL]

No problem, this clearly highlights that we had some missing test cases. I can come up with a fix.