stream: readall(): Make sure there's a trailing NUL char.

py/stream.c

@@ -289,6 +289,7 @@ STATIC mp_obj_t stream_readall(mp_obj_t self_in) {
     }
 
     vstr.len = total_size;
+    vstr.buf[total_size] = '\0';
     return mp_obj_new_str_from_vstr(STREAM_CONTENT_TYPE(o->type->stream_p), &vstr);
 }