dgram: fix assertion on bad send() arguments

Add range checks for the offset, length and port arguments to
dgram.Socket#send().  Fixes the following assertion:

    node: ../../src/udp_wrap.cc:264: static v8::Handle<v8::Value>
    node::UDPWrap::DoSend(const v8::Arguments&, int): Assertion
    `offset < Buffer::Length(buffer_obj)' failed.

And:

    node: ../../src/udp_wrap.cc:265: static v8::Handle<v8::Value>
    node::UDPWrap::DoSend(const v8::Arguments&, int): Assertion
    `length <= Buffer::Length(buffer_obj) - offset' failed.

Interestingly enough, a negative port number was accepted until now but
silently ignored.  (In other words, it would send the datagram to a
random port.)

This commit exposed a bug in the simple/test-dgram-close test which
has also been fixed.

This is a back-port of commit 41ec6d05 from the master branch.

Fixes #6025.