syscall: reduce memory allocated by UTF16FromString

syscall: reduce memory allocated by UTF16FromString

The function allocated a buffer larger than needed.
Fixes #59967.

Signed-off-by: Frediano Ziglio <frediano.ziglio@nextdlp.com>
Change-Id: I5f30a135acf5f27d6c2ef4bc4abef5144da4dc94
Reviewed-on: https://go-review.googlesource.com/c/go/+/492575
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>