Fix write to uninitialized bytes for XCB event

Fix write to uninitialized bytes for XCB event

As the xcb_send_event man page [1] states,

  In order to properly initialize these bytes, we allocate 32 bytes
  even though we only need less for an xcb_configure_notify_event_t

This commit fixes the following Valgrind error, which could be
triggered by selecting a letter in LyX:

  ==12698== Syscall param writev(vector[...]) points to uninitialised byte(s)
  ==12698==    at 0x61F578D: __writev (writev.c:26)
  ==12698==    by 0x61F578D: writev (writev.c:24)
  ==12698==    by 0x4A83BFC: ??? (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
  ==12698==    by 0x4A83FD0: ??? (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
  ==12698==    by 0x4A84246: ??? (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
  ==12698==    by 0x4A84ACB: xcb_flush (in /usr/lib/x86_64-linux-gnu/libxcb.so.1.1.0)
  ==12698==    by 0x17C8F06: lyx::frontend::GuiApplication::nativeEventFilter(QByteArray const&, void*, long*) (GuiApplication.cpp:3366)
  ==12698==    by 0x5AA4EEE: QAbstractEventDispatcher::filterNativeEvent(QByteArray const&, void*, long*) (qabstracteventdispatcher.cpp:484)

[1] https://www.x.org/releases/current/doc/man/man3/xcb_send_event.3.xhtml