NetPBMWriter: Change destructor

Makes gcc 12.1 happier, i think it was a gcc bug anyway, but the new code is a bit "better" anyway, the warning i was getting is

In file included from utils/ImageOutputDev.cc:44: In destructor ‘virtual NetPBMWriter::~NetPBMWriter()’, inlined from ‘virtual NetPBMWriter::~NetPBMWriter()’ at goo/NetPBMWriter.h:41:31, inlined from ‘void ImageOutputDev::writeImage(GfxState*, Object*, Stream*, int, int, GfxImageColorMap*, bool)’ at utils/ImageOutputDev.cc:636:16: goo/NetPBMWriter.h:41:31: warning: array subscript ‘NetPBMWriter[0]’ is partly outside array bounds of ‘ImgWriter [2]’ [-Warray-bounds] 41 | ~NetPBMWriter() override {}; | ^ utils/ImageOutputDev.cc: In member function ‘void ImageOutputDev::writeImage(GfxState*, Object*, Stream*, int, int, GfxImageColorMap*, bool)’: utils/ImageOutputDev.cc:620:57: note: object of size 16 allocated by ‘operator new’ 620 | writer = new PNGWriter(PNGWriter::MONOCHROME); | ^ utils/ImageOutputDev.cc:623:51: note: object of size 16 allocated by ‘operator new’ 623 | writer = new PNGWriter(PNGWriter::GRAY); | ^ utils/ImageOutputDev.cc:627:52: note: object of size 16 allocated by ‘operator new’ 627 | writer = new PNGWriter(PNGWriter::RGB48); | ^ utils/ImageOutputDev.cc:630:50: note: object of size 16 allocated by ‘operator new’ 630 | writer = new PNGWriter(PNGWriter::RGB); | ^