Fixes NullPointerException for getMergedRegionAt(index) when the index provided is valid (ie, 2 merged regions, request for index 0 would return null).

git-svn-id: https://svn.apache.org/repos/asf/jakarta/poi/trunk@353023 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/hssf/model/Sheet.java

@@ -482,7 +482,7 @@ public class Sheet implements Model
     public MergeCellsRecord.MergedRegion getMergedRegionAt(int index)
     {
         //safety checks
-        if (index < numMergedRegions || mergedRecords.size() == 0)
+        if (index >= numMergedRegions || mergedRecords.size() == 0)
             return null;
             
         int pos = 0;