py/objarray: Fix array slice assignment when array is reallocated.

Addresses issue #1898.

py/objarray.c

@@ -439,6 +439,7 @@ STATIC mp_obj_t array_subscr(mp_obj_t self_in, mp_obj_t index_in, mp_obj_t value
                         // TODO: alloc policy; at the moment we go conservative
                         o->items = m_renew(byte, o->items, (o->len + o->free) * item_sz, (o->len + len_adj) * item_sz);
                         o->free = 0;
+                        dest_items = o->items;
                     }
                     mp_seq_replace_slice_grow_inplace(dest_items, o->len,
                         slice.start, slice.stop, src_items, src_len, len_adj, item_sz);

tests/basics/bytearray_slice_assign.py

@@ -47,6 +47,9 @@ b = bytearray(2)
 b[2:] = bytearray(10)
 print(b)
 
+b = bytearray(10)
+b[:-1] = bytearray(500)
+print(len(b), b[0], b[-1])
 
 # Assignment of bytes to array slice
 b = bytearray(2)