Fix uninitialised read in OperationDetail::set_status()

Setting the status and controlling the timing of operation details
initialised with OperationDetail(desc, status, font) uses a conditional
branch depending on the uninitialised variable this->status.

valgrind:
==28957== Conditional jump or move depends on uninitialised value(s)
==28957==    at 0x80E4287:
GParted::OperationDetail::set_status(GParted::OperationDetailStatus)
(OperationDetail.cc:77)

OperationalDetail.cc:
   75  void OperationDetail::set_status( OperationDetailStatus status )
   76  {
>> 77          if ( this ->status != STATUS_ERROR )
   78          {
   79                  switch ( status )
   80                  {

src/OperationDetail.cc

@@ -32,6 +32,7 @@ OperationDetail::OperationDetail()
 
 OperationDetail::OperationDetail( const Glib::ustring & description, OperationDetailStatus status, Font font )
 {
+	this ->status = STATUS_NONE; // prevent uninitialized member access
 	set_description( description, font ) ;
 	set_status( status ) ;