Fix error handling in sync_helper() and imap_sync_mailbox(). (closes #3817)

Fix error handling in sync_helper() and imap_sync_mailbox(). (closes #3817)

This patch is based on the one Richard Russon found in the Fedora package.

If an error occurs during one of the imap_exec() calls in
imap_sync_mailbox(), the mailbox could end up being closed.  This
would cause idata->ctx to be NULL.  Add a check in sync_helper() for
the case where idata->ctx == NULL.

In imap_sync_mailbox(), check the return value of sync_helper().  To
keep the code simple, change rc from being the sum of the calls to the
bitwise-OR of the calls.  (We only need to know if a single flag needs
to be updated, and bitwise-OR will detect negatives.)

Below the calls to sync_helper(), if the call to imap_exec() fails,
make sure rc is set to -1.