Make RsaOpenSsl::ExportParameters(true) on public keys.

The Windows version throws when private data export is requested of a public-only key.  The Unix version returned the structure with the private key portions as null.  This change makes the Unix version throw if the private parameters don't export (such as because it's a public-only key).

Author: bartonjs

src/Common/src/Interop/Unix/libcrypto/Interop.Rsa.cs

@@ -83,6 +83,16 @@ internal static unsafe RSAParameters ExportRsaParameters(SafeRsaHandle key, bool
                     rsaParameters.Q = ExtractBignum(rsaStructure->q, halfModulus);
                     rsaParameters.DQ = ExtractBignum(rsaStructure->dmq1, halfModulus);
                     rsaParameters.InverseQ = ExtractBignum(rsaStructure->iqmp, halfModulus);
+
+                    if (rsaParameters.D == null ||
+                        rsaParameters.P == null ||
+                        rsaParameters.DP == null ||
+                        rsaParameters.Q == null ||
+                        rsaParameters.DQ == null ||
+                        rsaParameters.InverseQ == null)
+                    {
+                        throw new CryptographicException(SR.Cryptography_CSP_NoPrivateKey);
+                    }
                 }
             }
             finally

src/System.Security.Cryptography.RSA/tests/ImportExport.cs

@@ -155,7 +155,6 @@ public static void MultiExport()
         }
 
         [Fact]
-        [ActiveIssue(1984, PlatformID.AnyUnix)]
         public static void PublicOnlyPrivateExport()
         {
             RSAParameters imported = new RSAParameters