DDMsgReader: Fix for bogus messages when changing to an empty sub-board

Fix: When changing to an empty sub-board from within the reader (either from
read mode or list mode), it now properly says there are no messages and exits,
rather than showing a list of bogus messages.
Unsure when this bug was introduced.
The version is now 1.46.
14 jobs for dd_msg_reader_chg_empty_subboard_fix in 9 minutes and 48 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Build
passed #32672
FreeBSD
jsdoor-freebsd

00:04:17

passed #32673
Linux
jsdoor-linux

00:06:41

passed #32675
FreeBSD
jsdoor-windows

00:01:54

passed #32667
FreeBSD
sbbs-freebsd

00:05:47

passed #32662
Linux
sbbs-linux

00:09:46

passed #32665
Windows
sbbs-windows

00:05:41

passed #32674
FreeBSD
sexpots-freebsd

00:00:19

passed #32663
Linux
sexpots-linux

00:00:22

passed #32666
Windows
sexpots-windows

00:00:33

passed #32670
FreeBSD
syncdraw-freebsd

00:00:33

passed #32671
Linux
syncdraw-linux

00:01:01

passed #32668
FreeBSD
syncterm-freebsd

00:01:15

passed #32664
Linux
syncterm-linux

00:02:56

passed #32669
FreeBSD
syncterm-windows

00:01:55