[PD] fix multi-transform View

- fixes bug 4581
  See the first issue reported here:
  https://forum.freecadweb.org/viewtopic.php?f=3&t=56093#p482553

- also fix issue that one could set 1 occurrence for polar patterns despite 2 are required at least

- also fix potential dereferencing null pointer (reported by MSVC)

src/Mod/PartDesign/Gui/TaskPolarPatternParameters.cpp

@@ -156,6 +156,7 @@ void TaskPolarPatternParameters::setupUI()
 
     ui->polarAngle->bind(pcPolarPattern->Angle);
     ui->spinOccurrences->setMaximum(INT_MAX);
+    ui->spinOccurrences->setMinimum(2);
     ui->spinOccurrences->bind(pcPolarPattern->Occurrences);
 
     ui->comboAxis->setEnabled(true);