PATCH: [perl #133988], Assertion failure

PATCH: [perl #133988], Assertion failure

This was due to not checking that a \N{} expanded to a single code point
in contexts where only that is legal.  This never could have worked
properly, though the assertion failure is new.

The diagnostic needs to be reworded to accommodate this new case, but
its too late in the 5.29 cycle to do that, so I'm changing just the
description, and will reword in 5.31, [perl #133996]