eg example_2004.dwg
```
Preview (31439)
-------------------
overall size: 31403 [RL]
num_pictures: 2 [RC]
        [0] Code: 1 [RC]
                Header data start: 0x1e7 [RL]
                Header data size: 80 [RL]
        [1] Code: 2 [RC]
                Header data start: 0x237 [RL]
                BMP size: 31304 [RL]
BMP offset: 119
ERROR: Preview overflow 119 + 0 > 31407
Warning: thumbnail.size mismatch: 31407 != 0
```
Wrong BMP preview logic
This is a GitHub Issue
repo:libredwg
owner:LibreDWG
Title : Wrong BMP preview logic
Issue date:

--- start body ---
eg example_2004.dwg
```
Preview (31439)
-------------------
overall size: 31403 [RL]
num_pictures: 2 [RC]
        [0] Code: 1 [RC]
                Header data start: 0x1e7 [RL]
                Header data size: 80 [RL]
        [1] Code: 2 [RC]
                Header data start: 0x237 [RL]
                BMP size: 31304 [RL]
BMP offset: 119
ERROR: Preview overflow 119 + 0 > 31407
Warning: thumbnail.size mismatch: 31407 != 0
```
--- end body ---

None