DecoderSelector: Fix catching polymorphic types by value.

  [...]/pv/subwindows/decoder_selector/item.cpp: In member function ‘std::shared_ptr<pv::subwindows::decoder_selector::DecoderCollectionItem> pv::subwindows::decoder_selector::DecoderCollectionItem::subItem(int) const’:
  [...]/pv/subwindows/decoder_selector/item.cpp:44:11: warning: catching polymorphic type ‘class std::out_of_range’ by value [-Wcatch-value=]
    } catch (out_of_range) {
             ^~~~~~~~~~~~
  [...]/pv/subwindows/decoder_selector/item.cpp: In member function ‘QVariant pv::subwindows::decoder_selector::DecoderCollectionItem::data(int) const’:
  [...]/pv/subwindows/decoder_selector/item.cpp:88:11: warning: catching polymorphic type ‘class std::out_of_range’ by value [-Wcatch-value=]
    } catch (out_of_range) {
             ^~~~~~~~~~~~