strings: avoid utf8.RuneError mangling in Split

strings: avoid utf8.RuneError mangling in Split

Split should only split strings and not perform mangling
of invalid UTF-8 into ut8.RuneError.
The prior behavior is clearly a bug since mangling is not
performed in all other situations (e.g., separator is non-empty).

Fixes #53511

Change-Id: I112a2ef15ee46ddecda015ee14bca04cd76adfbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/413715
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>