Will he who swore not to masturbate for a month, and said: "If I do it, O Allah, admit me to the Fire," enter the Fire, if semen was then released from him involuntarily after five days?

First, masturbation is forbidden and must be abandoned. However, if it does not lead to the emission of semen, then you have not broken an oath and no expiation is due from you. Your statement, "May Allah admit him to Hell," is considered a slip of the tongue (laghw) and does not constitute an oath. As for your question, "Will I enter Hell?", no specific individual is definitively declared to enter Hell or Paradise except through a textual evidence (nass shar'i); rather, knowledge of their fate is left to Allah.