Correct calculation of base64 encode buffer length.

Correct calculation of base64 encode buffer length.

In the previous formulation, it would first multiple then divide. It would then
promote to unsigned int. The formula would overflow for large len. For example,
needed = 2 when len == INT_MAX. In the revised formulation, it is promoted,
divided, then multiplied. The revised calculation avoids the overflow and
computes needed correctly over len in {0, 1, ..., INT_MAX}.  For len == INT_MAX,
needed is correctly computed as 2863311533.

Signed-off-by: Tom Carroll <incentivedesign@gmail.com>