Depending on the benefits provided by your Defined Benefit scheme and your level of AVCs, it is possible to overfund. However your Trustees/Scheme advisor should ensure that such is unlikely.
In reality as you get closer to retirement you should seek direction from the Trustees/Advisor so as to ensure that you do not overfund. Also, unless your DB scheme is proving close to maximum benefits (incl post retirement indexation, spouses pension etc) it is unlikely that you will overfund.
Also, whether your DB scheme continues for the next 25 years is also a moot point.
Finally, in paying AVCs you cannot technically do so solely in anticipation of early retirement. In calculating the maximum level of AVCs you can only do so based on the main scheme benefits and the normal retirement age.
If in doubt, ask your Trustees or Scheme advisor.