LibCore: Fix WSAEMSGSIZE error in pending_bytes with proper zero-init

In SocketWindows, the return value for the ioctl call was not
initialized to zero. This was causing test_udp in TesDNSResolver
to fail as UDPSocket::read_some() was incorrectly failing with
WSAEMSGSIZE due the result of pending_bytes being some
unspecified default value for an uninitialized unsigned long

Libraries/LibCore/SocketWindows.cpp

@@ -116,7 +116,7 @@ ErrorOr<size_t> PosixSocketHelper::pending_bytes() const
         return Error::from_windows_error(WSAENOTCONN);
     }
 
-    u_long value;
+    u_long value = 0;
     TRY(System::ioctl(m_fd, FIONREAD, &value));
     return value;
 }