detect: limit base64_decode `bytes` to 64KiB

detect: limit base64_decode `bytes` to 64KiB

Ticket: 7613

Avoids potential large per-thread memory allocation. A buffer with the
size of the largest decode_base64 buffer size setting would be allocated
per thread. As this was a u32, it could mean a per-thread 4GiB memory
allocation.

64KiB was already the built-in default for cases where bytes size wasn't
specified.