Avoid zero length array with LTC_BASE64_URL unset

[mkj]

If LTC_BASE64_URL is unset it creates a zero length array, which isn't allowed by some compilers

[sjaeckel]

I've implemented a different version of this patch. Can you please have a look whether this would also be acceptable for you?

https://github.com/libtom/libtomcrypt/tree/fix-b64-zero-length-array

[karel-m]

I think @sjaeckel 's patch https://github.com/libtom/libtomcrypt/commit/d20fdcc76d6b78059af4b847b428bd34972883ce is slightly better