[Sheet] Assert non-null pointer before using it

src/Mod/Spreadsheet/App/PropertySheet.cpp

@@ -524,12 +524,11 @@ void PropertySheet::setAlias(CellAddress address, const std::string &alias)
 
     const Cell * aliasedCell = getValueFromAlias(alias);
     Cell * cell = nonNullCellAt(address);
+    assert(cell != 0);
 
     if (aliasedCell != 0 && cell != aliasedCell)
         throw Base::ValueError("Alias already defined.");
 
-    assert(cell != 0);
-
     /* Mark cells depending on this cell dirty; they need to be resolved when an alias changes or disappears */
     std::string fullName = owner->getFullName() + "." + address.toString();