## Motivation
This is motivated by a previously unfinished PR (#2332). In that PR, the
label -1 was changed to 255 in `BaseSegDataset`, which is correct.
However, it was changed at only one location. There is another location
in `mmseg/datasets/basesegdataset.py` where -1 was still being used that
was not converted to 255. I have now converted it to 255.
This is exactly same as a similar fix to the `master` branch via #2515 .
## Modification
I've simply converted the snipped
```python
if new_id != -1:
new_palette.append(palette[old_id])
```
to
```python
if new_id != 255:
new_palette.append(palette[old_id])
```
## Checklist
- [x] Pre-commit or other linting tools are used to fix the potential
lint issues.
- _I've fixed all linting/pre-commit errors._
- [x] The modification is covered by complete unit tests. If not, please
add more unit test to ensure the correctness.
- _No unit tests need to be added or were affected.
- [x] If the modification has potential influence on downstream
projects, this PR should be tested with downstream projects, like MMDet
or MMDet3D.
- _I don't think this change affects MMDet or MMDet3D._
- [x] The documentation has been modified accordingly, like docstring or
example tutorials.
- _This change fixes an existing bug and doesn't require modifying any
documentation/docstring._
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