I quote 16-3-21 on occasions here, but you left out subsection (b). Furthermore you need to look at OCGA as it pertains to totality of circumstances. If you in your illicit carry of a firearm, use said firearm even in self defense you muddy the water greatly.
Subsection (b) is irrelevant, as it basically states that this is no defense if you're in the commission of a felony, or are the aggressor. Also, carrying a weapon without a license is NOT a felony.