Fix generation of padding message before encrypting Elgamal in pgcrypto

Fix generation of padding message before encrypting Elgamal in pgcrypto

fe0a0b5, which has added a stronger random source in Postgres, has
introduced a thinko when creating a padding message which gets encrypted
for Elgamal.  The padding message cannot have zeros, which are replaced
by random bytes.  However if pg_strong_random() failed, the message
would finish by being considered in correct shape for encryption with
zeros.

Author: Tom Lane
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/20186.1546188423@sss.pgh.pa.us
Backpatch-through: 10