Initialize variable in thunar_tree_view_model_dir_files_changed
This fixes an uninitialized value warning in valgrind.
I noticed this issue while testing !433 (merged) and copying a file.
Conditional jump or move depends on uninitialised value(s)
at 0x21CADE: thunar_tree_view_model_dir_files_changed (thunar-tree-view-model.c:2810)
by 0x576E6BF: g_closure_invoke (gclosure.c:832)
by 0x579CA35: signal_emit_unlocked_R.isra.0 (gsignal.c:3980)
by 0x578DA41: signal_emit_valist_unlocked (gsignal.c:3612)
by 0x578DC76: g_signal_emit_valist (gsignal.c:3355)
by 0x578DD33: g_signal_emit (gsignal.c:3675)
by 0x18FA81: _thunar_folder_files_update_timeout (thunar-folder.c:595)
by 0x56693ED: g_timeout_dispatch (gmain.c:5121)
by 0x5667F68: g_main_dispatch.lto_priv.0 (gmain.c:3476)
by 0x56C63A6: UnknownInlinedFun (gmain.c:4284)
by 0x56C63A6: g_main_context_iterate_unlocked.isra.0 (gmain.c:4349)
by 0x5666161: g_main_context_iteration (gmain.c:4414)
by 0x5512B65: g_application_run (gapplication.c:2577)
by 0x148442: main (main.c:86)