Don't crash when trying to connect to an non-SSL service using SSL

Fixes issue 3798

src/com/fsck/k9/activity/setup/AccountSetupCheckSettings.java

@@ -153,9 +153,17 @@ public class AccountSetupCheckSettings extends K9Activity implements OnClickList
                         afe.getMessage() == null ? "" : afe.getMessage());
                 } catch (final CertificateValidationException cve) {
                     Log.e(K9.LOG_TAG, "Error while testing settings", cve);
-                    acceptKeyDialog(
-                        R.string.account_setup_failed_dlg_certificate_message_fmt,
-                        cve);
+
+                    // Avoid NullPointerException in acceptKeyDialog()
+                    if (TrustManagerFactory.getLastCertChain() != null) {
+                        acceptKeyDialog(
+                            R.string.account_setup_failed_dlg_certificate_message_fmt,
+                            cve);
+                    } else {
+                        showErrorDialog(
+                                R.string.account_setup_failed_dlg_server_message_fmt,
+                                (cve.getMessage() == null ? "" : cve.getMessage()));
+                    }
                 } catch (final Throwable t) {
                     Log.e(K9.LOG_TAG, "Error while testing settings", t);
                     showErrorDialog(

src/com/fsck/k9/mail/store/TrustManagerFactory.java

@@ -76,6 +76,8 @@ public final class TrustManagerFactory {
 
         public void checkServerTrusted(X509Certificate[] chain, String authType)
         throws CertificateException {
+            // FIXME: Using a static field to store the certificate chain is a bad idea. Instead
+            // create a CertificateException subclass and store the chain there.
             TrustManagerFactory.setLastCertChain(chain);
             try {
                 defaultTrustManager.checkServerTrusted(chain, authType);