[libcxx][ios] initialize __fill_val_ in _FillHelper (#110279) This is a small fix to https://github.com/llvm/llvm-project/pull/89305. In the `__init` function of `_FillHelper`, `__fill_val_` was left uninitialized. This worked for the implementation in the PR because we always checked `__set_` before trying to read it, and would initialize if it was unset. However it turns out in earlier versions of the header (at least on AIX which followed this path), we do a read of `__fill_val_` even if `__set_` was false before initializing, to check if it matched the sentinel value, so this causes undesired behaviour and UB.
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