Avoid a theoretical integer overflow in base64_encode()

Avoid a theoretical integer overflow in base64_encode()

If base64_encode() were to be used with a huge data array, the
previous version could have resulted in overwriting the allocated
buffer due to an integer overflow as pointed out in
http://www.freebsd.org/cgi/query-pr.cgi?pr=137484. However, there
are no know use cases in hostapd or wpa_supplicant that would do that.
Anyway, the recommended change looks reasonable and provides additional
protection should the base64_encode() function be used for something
else in the future.