Fix g_hash_table_get_keys leak

Leak report

Direct leak of 24 byte(s) in 1 object(s) allocated from:
    #0 0x59b2a1f818b9 in malloc (/usr/asan/bin/thunar+0x4988b9)
    #1 0x7054d1fd5062 in g_malloc glib/gmem.c:100:13
    #2 0x7054d1fb2cd6 in g_list_prepend glib/glist.c:253:14
    #3 0x7054d1f95b42 in g_hash_table_get_keys glib/ghash.c:2248:18
    #4 0x59b2a239a2ae in _thunar_tree_view_model_folder_error thunar/thunar-tree-view-model.c:2468:28
    #5 0x7054d2175d30 in g_closure_invoke gobject/gclosure.c:833:7
    #6 0x7054d21b012e in signal_emit_unlocked_R gobject/gsignal.c:3887:8
    #7 0x7054d21ad98c in signal_emit_valist_unlocked gobject/gsignal.c:3519:7
    #8 0x7054d21ae910 in g_signal_emit_valist gobject/gsignal.c:3262:7
    #9 0x7054d21ae910 in g_signal_emit gobject/gsignal.c:3582:3
    #10 0x59b2a212e907 in thunar_folder_error thunar/thunar-folder.c:673:3
[...snip...]

Steps to reproduce

• #1441 (comment 94585)

Testing

• No issues were detected with LSan, ASan, or Valgrind when testing the patch