process_pipeline: replace sprintf with snprintf

to avoid potential buffer overflows

src/main.c

@@ -1413,9 +1413,10 @@ activate(GtkApplication *app, gpointer data)
         // Initialize the postprocessing gsetting to the old processor if
         // it was not set yet
         if (setting_postproc == NULL || setting_postproc[0] == '\0') {
+		const int size = 512;
                 printf("Initializing postprocessor gsetting\n");
-                setting_postproc = malloc(512);
-                if (!mp_process_find_processor(setting_postproc, "postprocess.sh")) {
+                setting_postproc = malloc(size);
+                if (!mp_process_find_processor(setting_postproc, size, "postprocess.sh")) {
                         printf("No processor found\n");
                         exit(1);
                 }