[RenderScript] fix Allocation.copyToFieldPacker

Change-Id: Iaba3333f9741238745fdb5c94f1b27b1015d50b2
diff --git a/rs/java/android/renderscript/Allocation.java b/rs/java/android/renderscript/Allocation.java
index 4fa2c81..2ce11a2 100644
--- a/rs/java/android/renderscript/Allocation.java
+++ b/rs/java/android/renderscript/Allocation.java
@@ -1448,7 +1448,7 @@
         }
 
         final byte[] data = fp.getData();
-        int data_length = fp.getPos();
+        int data_length = data.length;
         int eSize = mType.mElement.mElements[component_number].getBytesSize();
         eSize *= mType.mElement.mArraySizes[component_number];