/*
 * Reads one character from the stream, returning the byte read.
 *
 *    reader = Bzip2::Reader.new Bzip2.compress('ab')
 *    reader.getc # => 97
 *    reader.getc # => 98
 *    reader.getc # => nil
 *
 * @return [Integer, nil] the byte value of the character read or +nil+ if eoz
 *    has been reached
 */
static VALUE bz_reader_getc(VALUE obj) {