sanitizeUtf8 and ensureValidUtf8 only checked continuation-byte bits, so
overlong encodings (E0 80 80), UTF-16 surrogates (ED A0 80) and values
above U+10FFFF (F4 90 80 80) passed through. They exist to satisfy
proto3 string fields (proto_serializer.cpp calls them for every string),
which reject exactly those sequences, so a document containing them could
make a peer fail to parse the message. Share one well-formedness check
with sanitizeUtf8Strict; the lenient variant still replaces bytes with
'?'.
