fdo#35728: fix GridControl crash
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

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core