fix hexdump() when using an offset with -1 as length (= full length)

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/util/GB.java

@@ -126,7 +126,7 @@ public class GB {
 
     public static String hexdump(byte[] buffer, int offset, int length) {
         if (length == -1) {
-            length = buffer.length;
+            length = buffer.length - offset;
         }
         final char[] hexArray = "0123456789ABCDEF".toCharArray();
         char[] hexChars = new char[length * 2];