Is the fast of someone who had a wet dream a quarter of an hour before Fajr, and the Fajr adhan was called while he had not yet performed ghusl, valid?

Your fast is valid even if you did not perform ghusl until after Fajr prayer time began. If someone in a state of janabah does not perform ghusl until the time for Fajr prayer has entered, their fast is valid. It is important to note the absolute prohibition of masturbation and the health risks associated with it.