[Mono-list] Patch for crash in async operations

[Elan Feingold]

Any time a Worker was created with a null callback, for example:

network.BeginWrite (buffer, offset, size, null, null);

This would result in:

Unhandled Exception: System.NullReferenceException: A null value was
found where an object instance was required
in <0x000b7> 00 .Worker:End ()
in <0x00068> 00 .Worker:Send ()
in <0x0005b> 01 System.MulticastDelegate:invoke_void ()

This is because the code in Socket.cs wasn't checking for a null
callback.

My Changelog entry:

* Socket.cs: Check for a null callback before calling it.

Best regards,

-elan

patch
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