Explicitly check for empty ASN.1 strings in d2i_ECPrivateKey

The old code implicitly relies on the ASN.1 code returning a \0-prefixed buffer
when the buffer length is 0. Change this to verify explicitly that the ASN.1 string
has positive length.

Reviewed-by: Dr Stephen Henson <steve@openssl.org>
(cherry picked from commit 82dc08de54ce443c2a9ac478faffe79e76157795)