bin_to_base64() (src/base64.c) can experience an integer overflow and
subsequent under allocation, leading to a write beyond bounds. The bug can
occur only in 32-bit builds.

The only problematic use case is ssh_get_fingerprint_hash() in case the API is
(mis)used and a libssh consumer passes in an unexpectedly large input buffer.

As a mitigation, the function bin_to_base64() is adjusted to not allow inputs
larger than 256MB, which is aligned with other functions that process user
input.