InputStream integer return fixes.

Fix one condition where read returns a large negative integer instead of the correct byte value. 
Fix a second condition where checkFromBuffer returns a negative number and triggers a serial read before the buffer is depleted.

For example, if 0xFE is read, 0xFFFFFFFE is returned from the buffer which is out of the byte range.

usbserial/src/main/java/com/felhr/usbserial/SerialInputStream.java

@@ -42,7 +42,7 @@ public int read()
         int ret = device.syncRead(buffer, timeout);
         if(ret >= 0) {
             bufferSize = ret;
-            return buffer[pointer++];
+            return buffer[pointer++] & 0xff;
         }else {
             return -1;
         }
@@ -68,7 +68,7 @@ public void setTimeout(int timeout) {
 
     private int checkFromBuffer(){
         if(bufferSize > 0 && pointer < bufferSize){
-            return buffer[pointer++];
+            return buffer[pointer++] & 0xff;
         }else{
             pointer = 0;
             bufferSize = -1;