Apply (modified) patch #9552: tftp_cleanup() for cleanup and filename buffer zeroing to prevent filename bugs

... without the memset to zero out the filename

src/include/lwip/apps/tftp_server.h

@@ -86,6 +86,7 @@ struct tftp_context {
 };
 
 err_t tftp_init(const struct tftp_context* ctx);
+void tftp_cleanup(void);
 
 #ifdef __cplusplus
 }