Initialize an empty message map in the message store on initialization

This addresses occasional crashes when a `MessageInfo` event reached the message
store before `DirectoryContents`, particularly on slower (imap) accounts.

lib/msgstore.go

@@ -48,6 +48,7 @@ func NewMessageStore(worker *types.Worker,
 	return &MessageStore{
 		Deleted:  make(map[uint32]interface{}),
 		DirInfo:  *dirInfo,
+		Messages: make(map[uint32]*models.MessageInfo),
 
 		selected:        0,
 		bodyCallbacks:   make(map[uint32][]func(io.Reader)),