Fix memory leak in DecoderMenu

==214004== 1,536 (16 direct, 1,520 indirect) bytes in 1 blocks are definitely lost in loss record 6,288 of 6,442
==214004==    at 0x4C2EE0B: malloc (vg_replace_malloc.c:299)
==214004==    by 0x67D8435: g_malloc (in /usr/lib64/libglib-2.0.so.0.5600.3)
==214004==    by 0x67F0056: g_slice_alloc (in /usr/lib64/libglib-2.0.so.0.5600.3)
==214004==    by 0x67F14FA: g_slist_copy_deep (in /usr/lib64/libglib-2.0.so.0.5600.3)
==214004==    by 0x5584FC: pv::widgets::DecoderMenu::DecoderMenu(QWidget*, bool) (decodermenu.cpp:33)
==214004==