split: prevent opening split when no messages are selected

The [v]split command panics when it is run with no message selected, or
when messages aren't loaded. Check for a valid selected message before
creating a split, and report an error if one isn't selected.

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

commands/account/split.go

@@ -55,9 +55,7 @@ func (Split) Execute(aerc *widgets.Aerc, args []string) error {
 		n = 0
 	}
 	if args[0] == "split" {
-		acct.Split(n)
-		return nil
+		return acct.Split(n)
 	}
-	acct.Vsplit(n)
-	return nil
+	return acct.Vsplit(n)
 }