openssl: Allow squeezing multiple times from SHAKE128/256 XOFs

openssl: Allow squeezing multiple times from SHAKE128/256 XOFs

OpenSSL currently doesn't support squeezing bytes out of an XOF multiple
times.  Unfortunately, EVP_DigestFinalXOF() completely resets the context
and later calls not simply fail, they cause a null-pointer dereference in
libcrypto.  This fixes the crash at the cost of repeating initializing
the whole state and allocating too much data for subsequent calls.

There is an open issue and PR that might add a function that allows
squeezing more data from an XOF in a future version of OpenSSL.