Fix native resource leaks in Keychain (#184)

- GetAllSigningIdentities: CFRelease the CFDataRef returned by
  SecCertificateCopyData (Copy semantics = +1 retain count).
- GetAllSigningCertificates: Same CFDataRef leak fix.
- FindInternetUserNameAndPassword: Free passwordData with
  SecKeychainItemFreeContent and release the item ref.
- FindInternetPassword(string,...): Same leak fix.
- FindInternetPassword(Uri): Free passwordData and align cleanup.

All three FindInternet* methods now use try/finally to guarantee native
resources are freed even if GetUsernameFromKeychainItemRef or marshaling
throws.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rmarinho

Xamarin.MacDev/Keychain.cs

@@ -459,6 +459,7 @@ public IList<AppleCodeSigningIdentity> GetAllSigningIdentities ()
 				if (SecIdentityCopyCertificate (itemRef, out certRef) == OSStatus.Ok) {
 					var data = SecCertificateCopyData (certRef);
 					var rawData = CFDataGetBytes (data);
+					CFRelease (data);
 
 					if (rawData != null) {
 						try {
@@ -520,6 +521,7 @@ public IList<X509Certificate2> GetAllSigningCertificates ()
 				if (SecIdentityCopyCertificate (itemRef, out certRef) == OSStatus.Ok) {
 					var data = SecCertificateCopyData (certRef);
 					var rawData = CFDataGetBytes (data);
+					CFRelease (data);
 
 					if (rawData != null) {
 						try {
@@ -943,9 +945,14 @@ public unsafe Tuple<string, string> FindInternetUserNameAndPassword (Uri uri)
 			if (result != OSStatus.Ok)
 				return null;
 
-			var username = GetUsernameFromKeychainItemRef (item);
-
-			return Tuple.Create (username, Marshal.PtrToStringAuto (passwordData, (int) passwordLength));
+			try {
+				var username = GetUsernameFromKeychainItemRef (item);
+				var password = Marshal.PtrToStringAuto (passwordData, (int) passwordLength);
+				return Tuple.Create (username, password);
+			} finally {
+				SecKeychainItemFreeContent (IntPtr.Zero, passwordData);
+				CFRelease (item);
+			}
 		}
 
 		public unsafe Tuple<string, string> FindInternetPassword (string serverName = "", string securityDomain = "", string accountName = "", string path = "", ushort port = 0)
@@ -969,9 +976,14 @@ public unsafe Tuple<string, string> FindInternetPassword (string serverName = ""
 			if (result != OSStatus.Ok)
 				return null;
 
-			var username = GetUsernameFromKeychainItemRef (item);
-
-			return Tuple.Create (username, Marshal.PtrToStringAuto (passwordData, (int) passwordLength));
+			try {
+				var username = GetUsernameFromKeychainItemRef (item);
+				var password = Marshal.PtrToStringAuto (passwordData, (int) passwordLength);
+				return Tuple.Create (username, password);
+			} finally {
+				SecKeychainItemFreeContent (IntPtr.Zero, passwordData);
+				CFRelease (item);
+			}
 		}
 
 		public string FindInternetPassword (Uri uri)
@@ -996,12 +1008,15 @@ public string FindInternetPassword (Uri uri)
 														  (uint) user.Length, user, (uint) path.Length, path, (ushort) uri.Port,
 														  0, auth, out passwordLength, out passwordData, ref item);
 
-			CFRelease (item);
-
-			if (result != OSStatus.Ok)
-				return null;
+			try {
+				if (result != OSStatus.Ok)
+					return null;
 
-			return Marshal.PtrToStringAuto (passwordData, (int) passwordLength);
+				return Marshal.PtrToStringAuto (passwordData, (int) passwordLength);
+			} finally {
+				SecKeychainItemFreeContent (IntPtr.Zero, passwordData);
+				CFRelease (item);
+			}
 		}
 	}