[LLVM] Don't memcpy from a null pointer for empty byte-array properties (#23077)
## Problem
`PropertyValue::PropertyValue(const byte *Data, SizeTy DataBitSize)` in
`llvm/lib/Support/PropertySetIO.cpp` ends with:
```cpp
// Append data.
std::memcpy(Val.ByteArrayVal + SizeFieldSize, Data, DataSize);
```
`Data` is null whenever the property value is an empty container: the
templated constructor in `llvm/include/llvm/Support/PropertySetIO.h`
forwards `Data.data()`, and for an empty
`std::vector<char>`/`SmallVector` that is `nullptr`. `memcpy`'s
parameters are declared `__attribute__((nonnull))`, so `memcpy(dst,
nullptr, 0)` is undefined behaviour even though the length is zero
(https://en.cppreference.com/cpp/string/byte/memcpy)
UBSan diagnostic (build configured with
`-DLLVM_USE_SANITIZER=Address;Undefined`):
```
llvm/lib/Support/PropertySetIO.cpp:171:49: runtime error: null pointer passed as
argument 2, which is declared to never be null
/usr/include/string.h:44:28: note: nonnull attribute specified here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior
```
## Fix
Skip the copy when there is nothing to copy:
```cpp
// Append data. Data may be null when DataSize is zero, and memcpy declares
// its source as nonnull, so guard the call.
if (DataSize > 0)
std::memcpy(Val.ByteArrayVal + SizeFieldSize, Data, DataSize);
```
----
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
llvm
5af20e1f - [LLVM] Don't memcpy from a null pointer for empty byte-array properties (#23077)
