Also switch the EVP_CIPHER copy to cut down on how frequently we need to cast
back and forth.

BUG=22

Change-Id: I9af1e586ca27793a4ee6193bbb348cf2b28a126e
Reviewed-on: https://boringssl-review.googlesource.com/7689
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>