Rename 'vbuff' to the more appropriate 'randfile'.
Remove the uninitialised buffer PRNG seed.

Use instead some rand() filled buffer. It is a bug if even that is not
enough, so complain loudly with UnexpectedError() (but try to continue).

In any case these kinds of hacks shouldn't be used, so print a
LOG_LEVEL_INFO message if that is necessary (e.g. on chroots that don't
have /dev/urandom). Still can be avoided by using WORKDIR/randseed.
Seed C PRNG with weak data, seed the OpenSSL PRNG...

...with bytes from rand() only if system's source of entropy is good.
join: Don't add delimiter if only null-values are left

Looks like a nested loop that introduces polinomial complexity,
but the inner loop advances the outer loop as well.

Fixes Redmine #6552
Merge pull request #1944 from jimis/entropy

Do not seed PRNG with uninitialised stack buffer.
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