Fix fatal bug in wxGridSelection::DeselectBlock()

The selBlock reference could be used after the vector invalidation,
which resulted in using the wrong vector element at best and could
probably lead to a crash at worst.

Don't use it after invalidating the vector any longer.

This notably fixes a bug in row-or-column selection mode where the grid
wasn't updated visually after modifying its selection by deselecting a
single cell as soon as there were more than one selected row or column.

src/generic/gridsel.cpp

@@ -290,14 +290,15 @@ wxGridSelection::DeselectBlock(const wxGridBlockCoords& block,
             break;
         }
 
-        // remove the block
+        const wxGridBlockDiffResult result =
+            selBlock.Difference(canonicalizedBlock, splitOrientation);
+
+        // remove the block (note that selBlock, being a reference, is
+        // invalidated here and can't be used any more below)
         m_selection.erase(m_selection.begin() + n);
         n--;
         count--;
 
-        wxGridBlockDiffResult result =
-            selBlock.Difference(canonicalizedBlock, splitOrientation);
-
         for ( int i = 0; i < 2; ++i )
         {
             const wxGridBlockCoords& part = result.m_parts[i];