Fixed bug where tearing down a connection would cause loss of memory.

src/include/lwip/sys.h

@@ -68,6 +68,9 @@ struct sys_timeout {u8_t dummy;};
 
 #include "arch/sys_arch.h"
 
+/** Return code for timeouts from sys_arch_mbox_fetch and sys_arch_sem_wait */
+#define SYS_ARCH_TIMEOUT 0xffffffff
+
 typedef void (* sys_timeout_handler)(void *arg);
 
 struct sys_timeout {