[Bug #21705] Fix segfaults on Windows

It should check the type of the argument and coercion before
converting the encoding.

ext/socket/unixsocket.c

@@ -42,11 +42,12 @@ unixsock_path_value(VALUE path)
         }
     }
 #endif
+    path = rb_get_path(path);
 #ifdef _WIN32
     /* UNIXSocket requires UTF-8 per spec. */
     path = rb_str_export_to_enc(path, rb_utf8_encoding());
 #endif
-    return rb_get_path(path);
+    return path;
 }
 
 VALUE

test/socket/test_unix.rb

@@ -293,14 +293,18 @@ class TestSocket_UNIXSocket < Test::Unit::TestCase
     File.unlink path if path && File.socket?(path)
   end
 
-  def test_open_nul_byte
-    tmpfile = Tempfile.new("s")
-    path = tmpfile.path
-    tmpfile.close(true)
-    assert_raise(ArgumentError) {UNIXServer.open(path+"\0")}
-    assert_raise(ArgumentError) {UNIXSocket.open(path+"\0")}
-  ensure
-    File.unlink path if path && File.socket?(path)
+  def test_open_argument
+    assert_raise(TypeError) {UNIXServer.new(nil)}
+    assert_raise(TypeError) {UNIXServer.new(1)}
+    Tempfile.create("s") do |s|
+      path = s.path
+      s.close
+      File.unlink(path)
+      assert_raise(ArgumentError) {UNIXServer.open(path+"\0")}
+      assert_raise(ArgumentError) {UNIXSocket.open(path+"\0")}
+      arg = Struct.new(:to_path).new(path)
+      assert_equal(path, UNIXServer.open(arg) { |server| server.path })
+    end
   end
 
   def test_addr