crypto/cipher: panic when IV length does not equal block size in NewOFB

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>

src/crypto/cipher/ofb.go

@@ -19,7 +19,7 @@ type ofb struct {
 func NewOFB(b Block, iv []byte) Stream {
 	blockSize := b.BlockSize()
 	if len(iv) != blockSize {
-		return nil
+		panic("cipher.NewOFB: IV length must equal block size")
 	}
 	bufSize := streamBufferSize
 	if bufSize < blockSize {