Improve EVP_PKEY_sign documentation

Clarify the intended use of EVP_PKEY_sign. Make the code example compile.

Reviewed-by: Dr Stephen Henson <steve@openssl.org>
RT3061: slightly amend patch

Add an extra NULL dereference check

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
RT 3060: amend patch

Use existing error code SSL_R_RECORD_TOO_SMALL for too many empty records.

For ease of backporting the patch to release branches.

Reviewed-by: Bodo Moeller <bodo@openssl.org>