Do nothing in ImapConnection.close() if connection is not open

Under certain circumstances ImapFolderPusher can call
ImapConnection.close() twice. When using compression this will
lead to a NullPointerException inside ZOutputStream.close(). We're
ignoring all exceptions when closing a connection. So this shouldn't be
a problem. But (early versions of?) Android 8.1 shipped with a bug in
ART that shuts down the VM when a NullPointerException is triggered from
AOT/JIT(?) compiled code that uses a certain optimization. And we're
unlucky enough to trigger this bug.
Not closing the streams more than once should work around this bug.

See issue #2931

k9mail-library/src/main/java/com/fsck/k9/mail/store/imap/ImapConnection.java

@@ -715,6 +715,10 @@ class ImapConnection {
     }
 
     public void close() {
+        if (!open) {
+            return;
+        }
+
         open = false;
         stacktraceForClose = new Exception();