How did Jimmy Valentine manage to get our of jail without serving the full term?

Jimmy Valentine only serves ten months of a four year sentence for safe-cracking, but then he had expected to serve only three months at the longest. We're told that Jimmy has quite a few friends on the outside, and the implication is that they must be socially prominent individuals. After all, Jimmy receives a pardon from the Governor himself, a rare honor indeed.
When the warden meets with Jimmy before his release, the convict amuses the warden with his continued protestations of innocence. If Jimmy really had been set up for the crime which landed him in prison, says the warden, then maybe it was because he wouldn't provide an alibi for fear of compromising a prominent member of society. This would appear to suggest that, when he was arrested for his crime, Jimmy kept his mouth shut to protect a very important person. If indeed that is the case, then Jimmy's early release could be seen as a reward for his silence.