Fix internal `Wtf8Buf::extend_from_slice` check

[Borgerr]

Reviewing PR #126885 had some oversight regarding valid utf8 checks. This was graciously brought to our (meaning myself and @workingjubilee) attention by @yhx-12243 in this comment. Fix should really just be changing line 483 of wtf8.rs to setting self.is_known_utf8 to false, unless we decide there may be some merit to iterating again through the other buffer.

[Borgerr]

@rustbot label T-libs C-cleanup O-windows

[Borgerr]

@rustbot label -needs-triage