datasets/string: fix buffer overflow

datasets/string: fix buffer overflow

The size of encoded_data array and the maximum output length parameter
to Base64Encode function were incorrect leading to buffer overflow for
certain cases. The algorithm requires at least 5 bytes of space to even
convert a string of length 1.

Use BASE64_BUFFER_SIZE macro to correctly calculate this output length.
Set size of encoded_data array to the calculated output length.