fdo#35728: fix GridControl crash

 * various forms.OGridControlModel tests segfaulted
 * root cause is the migration to stl containers in DbGridControl
 * the old tools container returned the removed pointer or NULL causing no failure
 * a signed int was used as index while the functions returned a unsigned int
 * the GRID_COLUMN_NOT_FOUND magic value was out of the signed range

forms/qa/unoapi/knownissues.xcl

@@ -129,9 +129,3 @@ forms.OGridControlModel::com::sun::star::view::XSelectionSupplier
 
 ### i111333 ###
 forms.OImageControlControl::com::sun::star::awt::XControl
-
-### fd#35728 ###
-forms.OGridControlModel::com::sun::star::form::component::GridControl
-forms.OGridControlModel::com::sun::star::beans::XMultiPropertySet
-forms.OGridControlModel::com::sun::star::beans::XPropertySet
-forms.OGridControlModel::com::sun::star::beans::XFastPropertySet