If it decrypts something that turns out to start with a NUL byte,
then decrypt_desc_layer() will return 0 to indicate the length of
its result. But 0 also indicates an error, which causes the result
not to be freed by decrypt_desc_layer()'s callers.
Since we're trying to stabilize 0.3.2.x, I've opted for the simpler
possible fix here and made it so that an empty decrypted string will
also count as an error.
Fixes bug 24150 and OSS-Fuzz issue 3994.
The original bug was present but unreachable in 0.3.1.1-alpha. I'm
calling this a bugfix on 0.3.2.1-alpha since that's the first version
where you could actually try to decrypt these descriptors.
The new decryption function performs no decryption, skips the salt,
and doesn't check the mac. This allows us to fuzz the
hs_descriptor.c code using unencrypted descriptor test, and exercise
more of the code.
Related to 21509.
