Fix CRL conversion issue when already in PEM format

If downloaded CRL file is already in PEM format, openssl command will fail as it is always trying to convert from a DER-encoded CRL. 
This commit is for adding a test of the CRL format prior to running the openssl crl conversion. 

Note: as the openssl verify command then assumes that a .pem tmpfile has been generated by the conversion, there would be an issue when the conversion was not needed (i.e. CRL already PEM-encoded) as that .pem would be missing; therefore I also added a copy of the .crl file to a .crl.pem file before the optional conversion.

Author: teki69