combo-row: fix -Wmaybe-uninitialized warning
Obviously not actually a bug, but GCC ain't so smart when it comes to autocleanup.
../src/adw-combo-row.c: In function ‘adw_combo_row_init’:
/usr/include/glib-2.0/glib/gmacros.h:1211:17: warning: ‘factory’ may be used uninitialized in this function [-Wmaybe-uninitialized]
1211 | { if (_ptr) (cleanup) ((ParentName *) _ptr); } \
| ^
../src/adw-combo-row.c:251:34: note: ‘factory’ was declared here
251 | g_autoptr (GtkListItemFactory) factory;
| ^~~~~~~