Writing wav to stdout no longer works with v2.x

[joolswills]

With sidplayfp v1.4.4 (with Ubuntu 20.04.4), you could write a wav to stdout with -w-.

With v2.2.3 (Ubuntu 22.04 version) this no longer works, as the .wav extension is added and a file -.wav is created instead.

I assume this is related to the extension being automatically added before the check in

sidplayfp/src/audio/wav/WavFile.cpp

Line 173 in 1dcccd3

if (name.compare("-") == 0)

(or something like that).

[drfiemost]

I guess it broke with 09512ab, need to check for stdout in getFileName and avoid adding the extension in that case