[libcxx][ios] initialize __fill_val_ in _FillHelper (#110279)

This is a small fix to #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.

Author: daltenty

libcxx/include/ios

@@ -524,7 +524,10 @@ template <class _Traits>
 // Attribute 'packed' is used to keep the layout compatible with the previous
 // definition of the '__fill_' and '_set_' pair in basic_ios on AIX & z/OS.
 struct _LIBCPP_PACKED _FillHelper {
-  _LIBCPP_HIDE_FROM_ABI void __init() { __set_ = false; }
+  _LIBCPP_HIDE_FROM_ABI void __init() {
+    __set_      = false;
+    __fill_val_ = _Traits::eof();
+  }
   _LIBCPP_HIDE_FROM_ABI _FillHelper& operator=(typename _Traits::int_type __x) {
     __set_      = true;
     __fill_val_ = __x;