Avoid NullPointerException if an image cannot be read

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1674952 13f79535-47bb-0310-9956-ffa450edef68

src/ooxml/java/org/apache/poi/xslf/usermodel/XSLFImageRenderer.java

@@ -100,6 +100,10 @@ public class XSLFImageRenderer {
             return false;
         }
         
+        if(img == null) {
+        	return false;
+        }
+
         int iw = img.getWidth();
 		int ih = img.getHeight();