drivers: wifi: esp_at: fix ssid_len calucation for AP query.

The driver seems to be designed to use the very last byte of the
buffer(scan, connect), so null terminating the status query
might have unintended consequences.
However we should not use strlen to determine the ssid_len,
to avoid depending on the following buffer(bssid) to be zeroed.
Related to CID 316354

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>