Check for :EOF in mk_clos_stream_read_char

[yitzchak]

The Gray stream protocol defines :EOF as the proper return from stream-read-char on EOF. I left the check for NIL as this what ECL and CLASP do and it seems relatively harmless.

  STREAM-READ-CHAR  stream          [Generic Function]

    This reads one character from the stream.  It returns either a
    character object, or the symbol :EOF if the stream is at end-of-file.
    Every subclass of FUNDAMENTAL-CHARACTER-INPUT-STREAM must define a
    method for this function.

[yitzchak]

Fixed.