Assertion fails in multi-char regex match

Assertion fails in multi-char regex match

In '"s\N{U+DF}" =~ /\x{00DF}/i, the LHS folds to 'sss', the RHS to 'ss'.
The bug occurs when the RHS tries to match the first two es's, but that
splits the LHS \xDF character, which Perl doesn't know how to handle,
and the assertion got triggered.  (This is similar to [perl #72998].)

The solution adopted here is to disallow a partial character match,
as #72998 did as well.