Allow gifs with missing graphics control block extension terminator to match GDI+ behaviour

[hughbe]

In test_validData in testgifcodec.c

#if defined(USE_WINDOWS_GDIPLUS)
  BYTE graphicsControlBlockMissingTerminator[] = {'G', 'I', 'F', '8', '9', 'a', 3, 0, 5, 0, 0, 0, 0, '!', 0xF9, 0x04, 0, 0, 0, 0, ',', 0, 0, 0, 0, 3, 0, 5, 0, B8(10000000), 0, 0, 0, 255, 255, 255, 0x02, 0x06, 0x84, 0x03, 0x81, 0x9a, 0x06, 0x05, 0x00, ';'};
#endif

  // FIXME: it appears that GDI+ allows a graphics control extension block without a
  // terminating 0 byte.
#if defined(USE_WINDOWS_GDIPLUS)
  createFileSuccess (graphicsControlBlockMissingTerminator, 3, 5);
#endif

This may not be possible to fix, as we rely on giflib