extmod/moduos_dupterm: Reserve buffer bytearray object for dupterm.

Allocating it for each read/write operation is a memory fragmentation
hazard.

extmod/moduos_dupterm.c

@@ -69,9 +69,12 @@ STATIC mp_obj_t mp_uos_dupterm(mp_uint_t n_args, const mp_obj_t *args) {
         }
     } else {
         if (args[0] == mp_const_none) {
-            MP_STATE_PORT(term_obj) = NULL;
+            MP_STATE_PORT(term_obj) = MP_OBJ_NULL;
         } else {
             MP_STATE_PORT(term_obj) = args[0];
+            if (MP_STATE_PORT(dupterm_arr_obj) == MP_OBJ_NULL) {
+                MP_STATE_PORT(dupterm_arr_obj) = mp_obj_new_bytearray(1, "");
+            }
         }
         return mp_const_none;
     }

py/mpstate.h

@@ -143,6 +143,7 @@ typedef struct _mp_state_vm_t {
 
     #if MICROPY_PY_OS_DUPTERM
     mp_obj_t term_obj;
+    mp_obj_t dupterm_arr_obj;
     #endif
 
     #if MICROPY_PY_LWIP_SLIP