Functions like NewCBCDecrypter, NewCBCEncrypter, NewCFBDecrypter,
NewCFBEncrypter and NewCTR all panic when IV length does not equal block size.
This commit changes NewOFB to panic too, instead of returning nil silently.

Change-Id: Ic4d3ebfad79bb0cf4759fa1c1a400c1a8d043490
Reviewed-on: https://go-review.googlesource.com/61850Reviewed-by: Filippo Valsorda <hi@filippo.io>
Run-TryBot: Filippo Valsorda <hi@filippo.io>
TryBot-Result: Gobot Gobot <gobot@golang.org>