reader.cpp: some minor adjustments to GetFileName related code
reader.cpp: some minor adjustments to GetFileName related code

In the name of hunt for bug which turned out to be older version of
sjasmplus on the user's system, I think these small patches should not
hurt (although the issues fixed can't be even triggered by asm file I
think?).

z00m128/sjasmplus,z00m128/sjasmplus,z00m128/sjasmplus