Add dispose of X509Chain in SignedXml (#115909)

* Add dispose of X509Chain in SignedXml

* Change try finally to using

* Remove spaces

* Add braces for using block

Author: MaxPatri

src/libraries/System.Security.Cryptography.Xml/src/System/Security/Cryptography/Xml/SignedXml.cs

@@ -353,10 +353,13 @@ public bool CheckSignature(X509Certificate2 certificate, bool verifySignatureOnl
                 }
 
                 // Do the chain verification to make sure the certificate is valid.
-                X509Chain chain = new X509Chain();
-                chain.ChainPolicy.ExtraStore.AddRange(BuildBagOfCerts());
-                bool chainVerified = chain.Build(certificate);
-                SignedXmlDebugLog.LogVerifyX509Chain(this, chain, certificate);
+                bool chainVerified = false;
+                using (X509Chain chain = new X509Chain())
+                {
+                    chain.ChainPolicy.ExtraStore.AddRange(BuildBagOfCerts());
+                    chainVerified = chain.Build(certificate);
+                    SignedXmlDebugLog.LogVerifyX509Chain(this, chain, certificate);
+                }
 
                 if (!chainVerified)
                 {