examples/ct: clear UnhandledCriticalExtensions
examples/ct: clear UnhandledCriticalExtensions

Go 1.5 added an x509.Certificate.UnhandledCriticalExtensions field,
and moved error generation when this is non-empty from the Parse()
method to the Verify() method.

To preserve the existing lax certificate checking behaviour in the
CT example code, explicitly clear out this field before calling
Verify().

daviddrysdale/trillian,google/trillian,google/trillian,AlCutter/trillian,RJPercival/trillian,AlCutter/trillian,codingllama/trillian,gtank/trillian,RJPercival/trillian,codingllama/trillian,gdbelvin/trillian,Martin2112/trillian,Martin2112/trillian,gtank/trillian,Martin2112/trillian,daviddrysdale/trillian,gdbelvin/trillian,daviddrysdale/trillian,gdbelvin/trillian