Merge pull request dotnet/corefx#9131 from stephentoub/fix_clientcert_crash

Fix crash in HttpClient on Unix when using client certificates

Commit migrated from https://github.com/dotnet/corefx/commit/30eee0b348e6a2ed1c3c40f94c2b5d53e769b274

src/libraries/System.Net.Http/src/System/Net/Http/Unix/CurlHandler.ClientCertificateProvider.cs

@@ -108,8 +108,10 @@ private int TlsClientCertCallback(IntPtr ssl, out IntPtr certHandle, out IntPtr
                             if (!Interop.Ssl.SslAddExtraChainCert(sslHandle, dupCertHandle))
                             {
                                 EventSourceTrace("Failed to add extra chain certificate");
+                                dupCertHandle.Dispose(); // we still own the safe handle; clean it up
                                 return SuspendHandshake;
                             }
+                            dupCertHandle.SetHandleAsInvalid(); // ownership has been transferred to sslHandle; do not free via this safe handle
                         }
                     }