make server a bit more resiliant

internal/coordinator/coordinator.go

@@ -57,12 +57,24 @@ func (c *Coordinator) Coordinate(cmd *SessionCommand) *SessionCommandResult {
 			Result: SessionRespFound,
 		}
 	case SessionCmdJoin:
+		if c.Match == nil {
+			return &SessionCommandResult{
+				ID:     cmd.ID,
+				Result: SessionRespJoinError,
+			}
+		}
 		resp := c.Match.Join(cmd.ID)
 		return &SessionCommandResult{
 			ID:     cmd.ID,
 			Result: resp,
 		}
 	case SessionCmdLeave:
+		if c.Match == nil {
+			return &SessionCommandResult{
+				ID:     cmd.ID,
+				Result: SessionRespLeft,
+			}
+		}
 		c.Match.Leave(cmd.ID)
 		return &SessionCommandResult{
 			ID:     cmd.ID,