commands/account: Disable :view for deleted msgs

Allowing the user to view deleted messages creates all sorts of race
conditions. The most devious race condition is pv.source can be set to a
nil while another PartViewer is still running a goroutine in
attemptCopy.

Here is a trace when this happens.

goroutine 76 [running]: io.copyBuffer(0x7f8ad02641d0, 0xc00040f590, 0x0,
0x0, 0xc0007cc000, 0x8000, 0x8000, 0x0, 0x0, 0x8b3d60)
/usr/lib/go/src/io/io.go:402 +0x101 io.Copy(...)
/usr/lib/go/src/io/io.go:364
git.sr.ht/~sircmpwn/aerc/widgets.(*PartViewer).attemptCopy.func4(0xc00017efd0,
0xc0004da7c0) /home/keur/repos/aerc/widgets/msgviewer.go:576 +0x611
created by git.sr.ht/~sircmpwn/aerc/widgets.(*PartViewer).attemptCopy
/home/keur/repos/aerc/widgets/msgviewer.go:544 +0x144

We could add a guard in store.FetchBodyPart to only call the callback
when msg.Part.Reader != nil, but we still get a hanging pager. Therefore
it seems more reasonable to disable this completely.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

commands/account/view.go

@@ -30,7 +30,8 @@ func (ViewMessage) Execute(aerc *widgets.Aerc, args []string) error {
 	}
 	store := acct.Messages().Store()
 	msg := acct.Messages().Selected()
-	if msg == nil {
+	_, deleted := store.Deleted[msg.Uid]
+	if msg == nil || deleted {
 		return nil
 	}
 	viewer := widgets.NewMessageViewer(acct, aerc.Config(), store, msg)