mail-source-fetch-pop, as distributed with Pterodactyl Gnus 0.84,
wants to run user-provided prescript and postscript code when pulling
mail off of a POP server.  Unfortunately, the postscript code is
broken: it checks to see whether prescript is defined, and if it is,
runs postscript.  This breaks if postscript is undefined.  This patch
should fix it: