Justin Bieber jumped the gun in his deposition Friday ... and in  the process admitted he was taking Rx meds illegally.As you know, Justin  is being sued by a photog who claims the singer instructed his  bodyguard to beat him up.During the questioning, the photog's lawyer  asked Bieber if he had a prescription for Xanax.  Before Justin's lawyer  could object, the singer blurted out that he did not.Justin's lawyer  was clearly upset and told Bieber to hold his horses before  answering.It's a significant question ... because during Justin's DUI  arrest cops say he admitted his mom had given him Xanax, and he tested  positive for Xanax which is the basis for the DUI charge.It probably  won't impact the case.  If Bieber was under the influence it doesn't  matter whether he took it legally or not.But it wasn't the smartest  thing for him to say.