utf8: non-character U+fffe as invalid codepoint

Unicode non-characters [1] are guaranteed to never be used for a
character. The formerly used U+fffd however is a valid character - the
replacement character [2] correctly displayed by Qt5 as <?>.

[1] https://en.wikipedia.org/wiki/Universal_Character_Set_characters#Non-characters
[2] https://en.wikipedia.org/wiki/Specials_(Unicode_block)#Replacement_character

Issue #3483

repos/gems/src/server/terminal/main.cc

@@ -368,7 +368,7 @@ void Terminal::Main::_handle_input()
 
 			if (special_sequence)
 				_read_buffer.add(special_sequence);
-			else
+			else if (codepoint.valid())
 				_read_buffer.add(codepoint);
 		});
 	});