Fix NPE in case when we can't find the Document atom: throw an IllegalState instead

git-svn-id: https://svn.apache.org/repos/asf/jakarta/poi/trunk@353797 13f79535-47bb-0310-9956-ffa450edef68

src/scratchpad/src/org/apache/poi/hslf/usermodel/SlideShow.java

@@ -224,6 +224,12 @@ public class SlideShow
 		}
 	}
 
+	// Ensure we really found a Document record
+	// If we didn't, then the file is probably corrupt
+	if(documentRecord == null) {
+		throw new IllegalStateException("The PowerPoint file didn't contain a Document Record in its PersistPtr blocks. It is probably corrupt.");
+	}
+
 
 	// Now look for SlideListWithTexts in the most up-to-date Document Record
 	//