Fix a crash in Macro.to_string if a tree looks like a sigil but the function is not an atom

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: dazuma

lib/elixir/lib/macro.ex

@@ -695,7 +695,7 @@ defmodule Macro do
   defp module_to_string(atom, _fun) when is_atom(atom), do: inspect(atom, [])
   defp module_to_string(other, fun), do: call_to_string(other, fun)
 
-  defp sigil_call({func, _, [{:<<>>, _, _} = bin, args]} = ast, fun) when is_list(args) do
+  defp sigil_call({func, _, [{:<<>>, _, _} = bin, args]} = ast, fun) when is_atom(func) and is_list(args) do
     sigil =
       case Atom.to_string(func) do
         <<"sigil_", name>> ->

lib/elixir/test/elixir/macro_test.exs

@@ -290,6 +290,8 @@ defmodule MacroTest do
     assert Macro.to_string(quote do: ~R"123") == ~s/~R"123"/
     assert Macro.to_string(quote do: ~R"123"u) == ~s/~R"123"u/
     assert Macro.to_string(quote do: ~R"\n123") == ~s/~R"\\\\n123"/
+
+    assert Macro.to_string(quote do: Foo.bar(<<>>, [])) == "Foo.bar(<<>>, [])"
   end
 
   test "arrow to string" do