dgram: don't assert on send('string')

Raise a TypeError when the argument to send() or sendto() is anything
but a Buffer.

Fixes the following assertion:

  $ node -e 'require("dgram").createSocket("udp4").send("BAM")'
  node: ../../src/udp_wrap.cc:220: static v8::Handle<v8::Value>
  node::UDPWrap::DoSend(const v8::Arguments&, int): Assertion
  `Buffer::HasInstance(args[0])' failed.
  Aborted (core dumped)

Fixes #4496.

lib/dgram.js

@@ -165,6 +165,9 @@ Socket.prototype.send = function(buffer,
                                  callback) {
   var self = this;
 
+  if (!Buffer.isBuffer(buffer))
+    throw new TypeError('First argument must be a buffer object.');
+
   if (offset >= buffer.length)
     throw new Error('Offset into buffer too large');
 

test/simple/test-dgram-regress-4496.js

@@ -0,0 +1,32 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+// Remove this test once we support sending strings.
+
+var common = require('../common');
+var assert = require('assert');
+var dgram = require('dgram');
+
+// Should throw but not crash.
+var socket = dgram.createSocket('udp4');
+assert.throws(function() { socket.send('BAM', 0, 1, 1, 'host') }, TypeError);
+assert.throws(function() { socket.sendto('BAM', 0, 1, 1, 'host') }, TypeError);
+socket.close();