Style: Replaces uses of 0/NULL by nullptr (C++11)

Using clang-tidy's `modernize-use-nullptr`.
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

core/io/net_socket.cpp

@@ -30,12 +30,12 @@
 
 #include "net_socket.h"
 
-NetSocket *(*NetSocket::_create)() = NULL;
+NetSocket *(*NetSocket::_create)() = nullptr;
 
 NetSocket *NetSocket::create() {
 	if (_create)
 		return _create();
 
 	ERR_PRINT("Unable to create network socket, platform not supported");
-	return NULL;
+	return nullptr;
 }