Fix crash in Directory Opus/MultiCommander integration due to UIPI blocking WM_COPYDATA

[Copilot]

QuickLook crashes when run with different privilege levels than Directory Opus or MultiCommander due to Windows UIPI blocking WM_COPYDATA messages, leaving XML buffer uninitialized and triggering RapidXML assertion at line 1405.

Changes

DOpus.cpp

• Initialize pXmlBuffer = nullptr to prevent uninitialized access
• Validate buffer non-null before XML parsing and cleanup
• Add null/size checks in WM_COPYDATA handler before allocation
• Define MAX_BUFFER_SIZE (10MB) to prevent memory exhaustion

MultiCommander.cpp

• Add null check for pCurrentItemPath before dereferencing
• Validate COPYDATASTRUCT in WM_COPYDATA handler
• Apply same 10MB buffer size limit

Memory safety

• Reset pointers to nullptr after deletion
• Clean up existing buffers before new allocations
• Prevent use-after-free and double-free scenarios

Example

// Before: crash on null buffer
doc.parse<0>(pXmlBuffer);  // Assert fails if UIPI blocks message

// After: graceful handling
if (pXmlBuffer == nullptr)
    return;
doc.parse<0>(pXmlBuffer);

Both integrations now gracefully handle IPC failures instead of crashing when privilege levels differ.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Error in Directory Opus</issue_title>
<issue_description>It gives me this error when I click preview on any file or even folders in Directory Opus.

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Microsoft Visual C++ Runtime Library

Assertion failed!

Program: ...Ta\Local\Programs\QuickLook\QuickLook.Native64.Dll
File: D:\GitHub\QuickLook\QuickLook.Native\Qui...\rapidxml.Hpp
Line: 1405

Expression: text

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After many mistakes and suffering, I found out that this happens If one or the other program is running as an administrator.
I need to preview in Everything, which I need to run as an administrator. But if I run QuickLook as an administrator, I get an error in Directory Opus.</issue_description>

<agent_instructions>我也复现了只有当QL管理员启动用于Dpous会报错，不是管理员启动则不会！</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Error in Directory Opus #1781

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