libstdc++: Set _M_string_length before calling _M_dispose() [PR109703]

This always sets _M_string_length in the constructor for ranges of input
iterators, such as stream iterators.

We copy from the source range to the local buffer, and then repeatedly
reallocate a larger one if necessary. When disposing the old buffer,
_M_is_local() is used to tell if the buffer is the local one or not (and
so must be deallocated). In addition to comparing the buffer address
with the local buffer, _M_is_local() has an optimization hint so that
the compiler knows that for a string using the local buffer, there is an
invariant that _M_string_length <= _S_local_capacity (added for PR109299
via r13-6915-gbf78b43873b0b7).  But we failed to set _M_string_length in
the constructor taking a pair of iterators, so the invariant might not
hold, and __builtin_unreachable() is reached. This causes UBsan errors,
and potentially misoptimization.

To ensure the invariant holds, _M_string_length is initialized to zero
before doing anything else, so that _M_is_local() doesn't see an
uninitialized value.

This issue only surfaces when constructing a string with a range of
input iterator, and the uninitialized _M_string_length happens to be
greater than _S_local_capacity, i.e., 15 for the std::string
specialization.

libstdc++-v3/ChangeLog:

	PR libstdc++/109703
	* include/bits/basic_string.h (basic_string(Iter, Iter, Alloc)):
	Initialize _M_string_length.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Co-authored-by: Jonathan Wakely <jwakely@redhat.com>