fix byte to integer conversion in feedableinputstream

bramble-core/src/main/java/org/briarproject/bramble/mailbox/sessions/FeedableSyncInputStream.java

@@ -24,7 +24,7 @@ public class FeedableSyncInputStream extends InputStream {
 			}
 		}
 
-		return activeBuffer[activeBufferPointer++];
+		return activeBuffer[activeBufferPointer++] & 0xff;
 	}
 
 	private boolean hasBytes() {