UBSan: Avoid unsigned integer overflow in base64 encoding

UBSan: Avoid unsigned integer overflow in base64 encoding

Add a constraint on the base64 encoded buffer length to avoid an integer
overflow in the output length calculation.

common.c:1087:16: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'size_t' (aka 'unsigned long')

Signed-off-by: Jouni Malinen <j@w1.fi>