libcpp: allow UCS_LIMIT codepoints in UTF-8 strings

Fixes r14-1954 (libcpp: reject codepoints above 0x10FFFF, 2023-06-06)

libcpp/

	* charset.cc: Allow `UCS_LIMIT` in UTF-8 strings.

Reported-by: Damien Guibouret <damien.guibouret@partition-saving.com>
Signed-off-by: Ben Boeckel <ben.boeckel@kitware.com>