Fix a potential crash in rand_unix.c

Due to the dynamic allocation that was added to rand_pool_add_begin
this function could now return a null pointer where it was previously
guaranteed to succeed. But the return value of this function does
not need to be checked by design.

Move rand_pool_grow from rand_pool_add_begin to rand_pool_bytes_needed.
Make an allocation error persistent to avoid falling back to less secure
or blocking entropy sources.

Fixes: a6a66e4511ee ("Make rand_pool buffers more dynamic in their sizing.")

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9687)

(cherry picked from commit fa3eb248e29ca8031e6a14e8a2c6f3cd58b5450e)