No,
Its not possible to fire it if you cock the hammer and your thumb slips off for two reasons.
Number one, the rebound slide comes up as the hammer falls UNLESS YOU ARE PULLING ON THE TRIGGER.
Number two, unless the cylinder has rotated to the point where the hammer locks up, chances are it couldnt fire even then because the round in the cylinder's charge hole has to be in alignment with the firing pin hole.
Even if the cylinder rotated fast enough to lock in place before the hammer is cocked, the firing pin will not go all the way forward unless the trigger is pulled, and the rebound slide will go up and get between the firing pin and the frame.
But dont take my word for it, unload your wheelgun, hold it up to the light and look by at the space in the cylinder window and pull the hammer back and let it go forward.
With Ruger revolvers the transfer bar will not stay up to allow the hammer to strike the firing pin if the thumb slips while being cocked.
The only sort of gun I would worry about discharging this way would be old style Single actions that dont have any sort of modern internal safey mechanisms.
The reason Smith went with the Colt type frame mounted firing pins was one of two reasons.
Number one, to avoid primer material from binding the cylinder
or
Number two, to make a good gun cheaper.
You can guess which version I believe is the answer.
>>>I don't know if they got sued or went PC, but it was possible to accidentally fire the gun while cocking the hammer if the thumb slipped off it.
<<<