[libc++abi] Don't do pointer arithmetic on nullptr (#119520)
`nullptr + offset` is possible after `!is_virtual` branch.
Detected with check-cxxabi on configured with:
```
cmake -DLLVM_APPEND_VC_REV=OFF -GNinja \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_CCACHE_BUILD=ON \
-DLLVM_USE_LINKER=lld \
-DLLVM_ENABLE_ASSERTIONS=ON \
-DCMAKE_C_COMPILER=clang \
-DCMAKE_CXX_COMPILER=clang++ \
-DLIBCXXABI_USE_LLVM_UNWINDER=OFF \
-DCMAKE_INSTALL_PREFIX=/home/b/sanitizer-aarch64-linux-bootstrap-ubsan/build/libcxx_install_ubsan \
'-DLLVM_ENABLE_RUNTIMES=libcxx;libcxxabi;libunwind' \
-DLIBCXX_TEST_PARAMS=long_tests=False \
-DLIBCXX_INCLUDE_BENCHMARKS=OFF \
-DLLVM_USE_SANITIZER=Undefined \
'-DCMAKE_C_FLAGS=-fsanitize=undefined -fno-sanitize-recover=all -fno-sanitize=vptr' \
'-DCMAKE_CXX_FLAGS=-fsanitize=undefined -fno-sanitize-recover=all -fno-sanitize=vptr' \
/home/b/sanitizer-aarch64-linux-bootstrap-ubsan/build/llvm-project/llvm/../runtimes
********************
Failed Tests (2):
llvm-libc++abi-shared.cfg.in :: catch_null_pointer_to_object_pr64953.pass.cpp
llvm-libc++abi-shared.cfg.in :: catch_ptr_02.pass.cpp
```
GitOrigin-RevId: a54fce89fc8aff36c50e3a0ea2f92e1ab7093cf8
1 file changed
