dxf2dwg issue with DXF file with CLASSES section which hasn't clasess

Example DXF generated by AutoCAD R14:
```
...
  0
SECTION
  2
CLASSES
  0
ENDSEC
  0
SECTION
  2
TABLES
  0
TABLE
  2
...
```

dxf2dwg output:
```
> dxf2dwg ex1.dxf 
Reading DXF file ex1.dxf
ERROR: 2 CLASSES must be followed by 0 CLASS
ERROR: Failed to decode DXF file: ex1.dxf

READ ERROR 0x800 ex1.dxf
```