The traditional private key encryption algorithm doesn't function
properly if the IV length of the cipher is zero. These ciphers
(e.g. ECB mode) are not suitable for private key encryption
anyway.

(Imported from upstream's 4436299296cc10c6d6611b066b4b73dc0bdae1a6.)

Change-Id: I218c9c1d11274ef11b7c0cfce380521efa415215
Reviewed-on: https://boringssl-review.googlesource.com/7840
Reviewed-by: Adam Langley <agl@google.com>