bug 59881: fix D* functions for blank cells in headers in database; patch from Patrick Zimmermann

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1753346 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/ss/formula/functions/DStarRunner.java

@@ -193,6 +193,9 @@ public final class DStarRunner implements Function3Arg {
         final int width = db.getWidth();
         for(int column = 0; column < width; ++column) {
             ValueEval columnNameValueEval = db.getValue(0, column);
+            if(solveReference(columnNameValueEval) instanceof BlankEval) {
+                continue;
+            }
             String columnName = getStringFromValueEval(columnNameValueEval);
             if(name.equals(columnName)) {
                 resultColumn = column;