[llvm] [llvm] Fixed Demangle OutputBuffer (PR #138564)

[Dmitry Vasilyev via llvm-commits]

https://github.com/slydiman created https://github.com/llvm/llvm-project/pull/138564

The [llvm-clang-x86_64-expensive-checks-win](https://lab.llvm.org/buildbot/#/builders/14/builds/3075) buildbot has been broken by #133249. 
DemangleTests causes the exception 0x80000003 inside CRT in case of the debug build on Windows.

>From 31578669f02b267296bb4d0fa1cbc091a7869f0b Mon Sep 17 00:00:00 2001
From: Dmitry Vasilyev <dvassiliev at accesssoftek.com>
Date: Mon, 5 May 2025 22:30:12 +0400
Subject: [PATCH] [llvm] Fixed Demangle OutputBuffer

The [llvm-clang-x86_64-expensive-checks-win](https://lab.llvm.org/buildbot/#/builders/14/builds/3075) buildbot has been broken by #133249.
DemangleTests causes the exception 0x80000003 inside CRT in case of the debug build on Windows.
---
 llvm/include/llvm/Demangle/Utility.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/llvm/include/llvm/Demangle/Utility.h b/llvm/include/llvm/Demangle/Utility.h
index d59d74511dd4f..002a1f55467d6 100644
--- a/llvm/include/llvm/Demangle/Utility.h
+++ b/llvm/include/llvm/Demangle/Utility.h
@@ -136,6 +136,8 @@ class OutputBuffer {
 
   OutputBuffer &prepend(std::string_view R) {
     size_t Size = R.size();
+    if (!Size)
+      return *this;
 
     grow(Size);
     std::memmove(Buffer + Size, Buffer, CurrentPosition);