cff.cpp: Move check for postcard string to common code path
cff.cpp: Move check for postcard string to common code path

The string "CUD-FM-File - SEND A POSTCARD -" should be present
also in uncompressed files, I suppose (at least it is in the few
examples I've seen). Move the check that was executed for packed
files only into code that's common for both cases.

adplug/adplug,adplug/adplug