BUG grow_labels with overlap=False

to replicate

```
import mne
from mne.datasets import sample

data_path = sample.data_path()
fname = data_path + '/MEG/sample/sample_audvis-meg'
stc = mne.read_source_estimate(fname)
labels = mne.grow_labels('fsaverage', stc.get_peak(hemi='lh')[0], 1, 1, overlap=False)
```

see https://mne.discourse.group/t/potential-bug-with-grow-labels/2679/1