Set buffer correctly when deleting column in math grid

This is a followup to 837bcbb0.

Fixes bug #11623.

src/mathed/InsetMathGrid.cpp

@@ -896,6 +896,8 @@ void InsetMathGrid::delCol(col_type col)
 			tmpcellinfo.push_back(cellinfo_[i]);
 		}
 	swap(cells_, tmpcells);
+	// copying cells loses the buffer reference
+	setBuffer(*buffer_);
 	swap(cellinfo_, tmpcellinfo);
 
 	colinfo_.erase(colinfo_.begin() + col);