ca_revoke() gets called two ways. Directly from ca_opt() with keyname

ca_revoke() gets called two ways.  Directly from ca_opt() with keyname
set to the cert to revoke, and indirectly from ca_create() with a
keyname set to NULL.

ca_create() sets REQ_EXT so avoid setting it in ca_revoke() when keyname
is NULL and the crl database is being initialised.

Avoids "REQ_EXT already set" when creating a CA error introduced
in rev 1.44 which set REQ_EXT unconditionally in ca_revoke().