process_pipeline: replace sprintf with snprintf

to avoid potential buffer overflows

src/process_pipeline.h

@@ -37,7 +37,8 @@ struct mp_process_pipeline_state {
         bool control_focus;
 };
 
-bool mp_process_find_processor(char *script, char *filename);
+
+bool mp_process_find_processor(char *script, int size, char *filename);
 void mp_process_find_all_processors(GtkListStore *store);
 
 void mp_process_pipeline_start();