Avoid a copy when using RSA_PADDING_NONE.

RSA_PADDING_NONE is actually the important one for RSA_decrypt since OAEP isn't
used much and RSA_PKCS1_PADDING is unsafe to use due to timing constraints.
(The SSL stack uses RSA_PADDING_NONE and does the padding check separately.)

Change-Id: I5f9d168e7c34796a41bf01fc1878022742b63501
Reviewed-on: https://boringssl-review.googlesource.com/5641
Reviewed-by: Adam Langley <agl@google.com>