(a) Conduct which would otherwise be an offense is justified by reason of necessity to the extent permitted by common law when
(1) neither this title nor any other statute defining the offense provides exemptions or defenses dealing with the justification of necessity in the specific situation involved; and
(2) a legislative intent to exclude the justification of necessity does not otherwise plainly appear.
(b) The justification specified in (a) of this section is an affirmative defense.