Clear position when putting without offset

Moving to Java 13+ would allow us to do absolute puts without the
position changing.

core/src/main/java/org/jruby/RubyIOBuffer.java

@@ -1300,8 +1300,8 @@ private void bufferCopy(ThreadContext context, int offset, ByteList sourceBuffer
         } else {
             destBuffer.position(offset);
             destBuffer.put(sourceBuffer.getUnsafeBytes(), sourceBuffer.begin() + sourceOffset, length);
-            destBuffer.clear();
         }
+        destBuffer.clear();
     }
 
     @JRubyMethod(name = "get_string")