fw_cfg: prevent selector key conflict

Enforce a single assignment of data for each distinct selector key.
Signed-off-by: NGabriel Somlo <somlo@cmu.edu>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/nvram/fw_cfg.c

@@ -423,6 +423,7 @@ static void fw_cfg_add_bytes_read_callback(FWCfgState *s, uint16_t key,
     key &= FW_CFG_ENTRY_MASK;
 
     assert(key < FW_CFG_MAX_ENTRY && len < UINT32_MAX);
+    assert(s->entries[arch][key].data == NULL); /* avoid key conflict */
 
     s->entries[arch][key].data = data;
     s->entries[arch][key].len = (uint32_t)len;