msgstore: check if message index < 0, select 0 if so

Calling :prev without this check can cause the index to go below 0 if
the current index is smaller than the value being scrolled / incremented
by. This results in the email selector wrapping around from the top to
the bottom most email in the mailbox folder due to changes in ec150f0
'store: fix Select behaviour'.

Signed-off-by: akspecs <akspecs@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

lib/msgstore.go

@@ -676,7 +676,11 @@ func (store *MessageStore) NextPrev(delta int) {
 		return
 	}
 	idx := store.SelectedIndex() + delta
+	if idx < 0 {
+		store.Select(0)
+	} else {
 		store.Select(idx)
+	}
 	store.updateVisual()
 	nextResultIndex := len(store.results) - store.resultIndex - 2*delta
 	if nextResultIndex < 0 || nextResultIndex >= len(store.results) {