VSE: Fix memory leak when building proxies

With no valid strips selected, file_list is not freed.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D10191

source/blender/editors/space_sequencer/sequencer_proxy.c

@@ -168,13 +168,13 @@ static void seq_proxy_build_job(const bContext *C, ReportList *reports)
   }
   SEQ_CURRENT_END;
 
+  BLI_gset_free(file_list, MEM_freeN);
+
   if (!selected) {
     BKE_reportf(reports, RPT_WARNING, "Select movie or image strips");
     return;
   }
 
-  BLI_gset_free(file_list, MEM_freeN);
-
   if (selected && !WM_jobs_is_running(wm_job)) {
     G.is_break = false;
     WM_jobs_start(CTX_wm_manager(C), wm_job);