fix various bugs in BufferedReader.readLine

classpath/java/io/BufferedReader.java

@@ -38,16 +38,18 @@ public class BufferedReader extends Reader {
       }
 
       if (position >= limit) {
-        return sb.toString();
+        return sb.length() == 0 ? null : sb.toString();
       }
 
       for (int i = position; i < limit; ++i) {
         if (buffer[i] == '\n') {
-          sb.append(buffer, position, i);
+          sb.append(buffer, position, i - position);
           position = i + 1;
           return sb.toString();
         }
       }
+      sb.append(buffer, position, limit-position);
+      position = limit;
     }
   }