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.

(Imported from upstream's 7f7c05ca638c3cc6d261961fae439cd91e3c1d27)

Change-Id: Icc6c44b874bdcb02374016a36d209830d6162a8a