peek_char/1 is broken when reading from atom

peek_char/1 doesn't just peek, but actually consumes
characters when reading from a stream. For instance:

?- open(atom('abc'), read, IO), see(IO),
get0(C1), peek_char(_), get0(C2),
atom_codes(A,[C1,C2]), writeln(A),
seen, close(IO).

prints `ac' and 'b' is missing.

I am quite certain that this was not the case with the
old file_read/2 when it was reading from a string.