fix directory lookup nil

weed/shell/command_fs_cat.go

@@ -56,6 +56,9 @@ func (c *commandFsCat) Do(args []string, commandEnv *CommandEnv, writer io.Write
 		if err != nil {
 			return err
 		}
+		if respLookupEntry.Entry == nil {
+			return fmt.Errorf("file not found: %s", path)
+		}
 
 		return filer2.StreamContent(commandEnv.MasterClient, writer, respLookupEntry.Entry.Chunks, 0, math.MaxInt32)