Fortuna: trivial static variable cleanup

[cem]

Summary

Remove unnecessary use of function-local static variable. 32 bytes is
small enough to live on the stack.

[delphij]

This would increase stack usage for /dev/random read by 32 bytes, but I think it's small enough and make the code easier to reason for MP safety (the buffer is not meant to be shared by two different threads), so consider this a LGTM from secteam@ as long as there is no objection raised by September 1st.

[cem]

In D16937#361440, @delphij wrote:

This would increase stack usage for /dev/random read by 32 bytes, but I think it's small enough and make the code easier to reason for MP safety (the buffer is not meant to be shared by two different threads), so consider this a LGTM from secteam@ as long as there is no objection raised by September 1st.

Yeah, the buffer should not be shared and it is a tiny amount of stack space. Our /dev/random read stacks are not super deep. (This isn't a security issue or implementation flaw due to the exclusive RANDOM_RESEED mutex being held over all accesses.) Thanks!

[markm]

If it doesn't crash anything, then fine! I had stack problems at some point.