Fixed another "There is already an open DataReader" error

X509CertificateDatabase.Find(...) methods keep the DataReader open until
we break out of the foreach loop and CreateCmsSigner(record) invokes
another db command, so we need to break out of the foreach loop before
calling CreateCmsSigner(record).

Fixes #1142 (comment)

Author: jstedfast

MimeKit/Cryptography/DefaultSecureMimeContext.cs

@@ -559,6 +559,7 @@ CmsSigner CreateCmsSigner (X509CertificateRecord record, DigestAlgorithm digestA
 		protected override CmsSigner GetCmsSigner (MailboxAddress mailbox, DigestAlgorithm digestAlgo)
 		{
 			X509CertificateRecord domain = null;
+			X509CertificateRecord signer = null;
 
 			foreach (var record in dbase.Find (mailbox, DateTime.UtcNow, true, CmsSignerFields)) {
 				if (record.KeyUsage != X509KeyUsageFlags.None && (record.KeyUsage & DigitalSignatureKeyUsageFlags) == 0)
@@ -573,11 +574,15 @@ protected override CmsSigner GetCmsSigner (MailboxAddress mailbox, DigestAlgorit
 					continue;
 				}
 
-				return CreateCmsSigner (record, digestAlgo);
+				signer = record;
+				break;
 			}
 
-			if (domain != null)
-				return CreateCmsSigner (domain, digestAlgo);
+			// fall back to a domain-wide signing certificate if an explicit signer was not found
+			signer ??= domain;
+
+			if (signer != null)
+				return CreateCmsSigner (signer, digestAlgo);
 
 			throw new CertificateNotFoundException (mailbox, "A valid signing certificate could not be found.");
 		}