When shutting down the memsys5 memory allocator, be sure to clear the mutex

pointer in case the next startup does not use a mutex because it is configured
differently.

FossilOrigin-Name: d4e7e2d82321c12fe471ed49098828bc0ef78543

src/mem5.c

@@ -472,6 +472,7 @@ static int memsys5Init(void *NotUsed){
 */
 static void memsys5Shutdown(void *NotUsed){
   UNUSED_PARAMETER(NotUsed);
+  mem5.mutex = 0;
   return;
 }