Prevent invalid index when converting formula from A1 to R1C1 format when we detect what appears to be a range expression but that expression is not well formed. In this case, it was simply text with a colon in it that made it look like a range expression.

EPPlus/ExcelCellBase.cs

diff --git a/EPPlus/ExcelCellBase.cs b/EPPlus/ExcelCellBase.cs
index f5694c1..e7a8f25 100644
--- a/EPPlus/ExcelCellBase.cs
+++ b/EPPlus/ExcelCellBase.cs
@@ -353,7 +353,7 @@
             string result = "";
             result += RangeCellToR1C1_V1(cellValues[0], row, col, rowIncr, colIncr);
             result += ":";
-            result += RangeCellToR1C1_V1(cellValues[1], row, col, rowIncr, colIncr);
+            result += (cellValues.Length > 1) ? RangeCellToR1C1_V1(cellValues[1], row, col, rowIncr, colIncr) : "";
 
             // Return converted range expression
             return result;