[Frontend] Fix a small `unique_ptr` array access.
When the size of the array accessed here is zero, retrieving the
address of the zero-th element here is undefined. When the frontend
is linked against a libc++ that has the `unique_ptr` hardening in
[this commit](https://github.com/llvm/llvm-project/commit/18df9d23ea390eaa50b41f3083a42f700a2b0e39)
enabled, it traps here.
Instead, simply call `.get()` to retrieve the address of the
array, which works even when it is a zero-byte allocation.
