Eliminate null references when Dispose is called.

EPPlus/CellStore.cs

diff --git a/EPPlus/CellStore.cs b/EPPlus/CellStore.cs
index a4dd683..6536325 100644
--- a/EPPlus/CellStore.cs
+++ b/EPPlus/CellStore.cs
@@ -202,7 +202,9 @@
         internal PageIndex[] _pages;
         internal int PageCount;
 
-        public void Dispose()
+    public void Dispose()
+    {
+        if (_pages != null)
         {
             for (int p = 0; p < PageCount; p++)
             {
@@ -210,6 +212,7 @@
             }
             _pages = null;
         }
+    }
 
     }
     internal class PageIndex : IndexBase, IDisposable
@@ -1431,11 +1434,14 @@
         public void Dispose()
         {
             if(_values!=null) _values.Clear();
-            for(var c=0;c<ColumnCount;c++)
+            if (_columnIndex != null)
             {
-                if (_columnIndex[c] != null)
+                for (var c = 0; c < ColumnCount; c++)
                 {
-                    ((IDisposable)_columnIndex[c]).Dispose();
+                    if (_columnIndex[c] != null)
+                    {
+                        ((IDisposable)_columnIndex[c]).Dispose();
+                    }
                 }
             }
             _values = null;