instead of a type error, use a domain error

The preceding use of atom_length/2 already ensures that C has the
correct type (i.e., atom). However, its domain may still be wrong,
if its length is greater than 1.

src/prolog/lib/charsio.pl

@@ -61,7 +61,7 @@ get_single_char(C) :-
     (  var(C) -> '$get_single_char'(C)
     ;  C == end_of_file  -> '$get_single_char'(C)
     ;  atom_length(C, 1) -> '$get_single_char'(C)
-    ;  throw(error(type_error(in_character, C), get_char/1))
+    ;  throw(error(domain_error(in_character, C), get_char/1))
     ).