wifi/esp32: Fix overflow in SSID copy

In the previous code, strnlen could have returned WIFI_SSID_MAX_LEN, and
the following statement ensuring NUL termination would have written one
past the end of the array.

Replace this with code that ensures a NUL termination within the bounds
of the array and then use strlen to compute the length.

Signed-off-by: Keith Packard <keithp@keithp.com>