ct_hammer: don't re-use the same error (#518)
ct_hammer: don't re-use the same error (#518)

Triggers a race as multiple threads write to the factory's
err variable.
daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go,RJPercival/certificate-transparency-go