PDFBOX-5660: optimize, as suggested by Valery Bokov, closes #361

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1930208 13f79535-47bb-0310-9956-ffa450edef68

Author: THausherr

tools/src/main/java/org/apache/pdfbox/tools/imageio/ImageIOUtil.java

@@ -396,10 +396,11 @@ private static void setDPI(IIOMetadata metadata, int dpi, String formatName)
         IIOMetadataNode child;
 
         child = getOrCreateChildNode(dimension, "HorizontalPixelSize");
-        child.setAttribute("value", Double.toString(res));
+        String stringRes = Double.toString(res);
+        child.setAttribute("value", stringRes);
 
         child = getOrCreateChildNode(dimension, "VerticalPixelSize");
-        child.setAttribute("value", Double.toString(res));
+        child.setAttribute("value", stringRes);
 
         metadata.mergeTree(MetaUtil.STANDARD_METADATA_FORMAT, root);
     }