`Image::fill_rect` may lead to silent crash

### Tested versions

Reproducible 4.6 stable and before

### System information

Windows 11

### Issue description

I've detected that sometimes (rare) this method leads to Access Violation Exception when used through GDExtension...

### Steps to reproduce

I need to prepare a test project, but for now, try to create and modify an Image using that method several thousand times in GDExtension.

### Minimal reproduction project (MRP)

Not now, maybe later...

Start by preparing the promised minimal reproduction project and repeatedly calling Image::fill_rect through GDExtension on Windows 11. Investigate the access violation under the reported conditions; done means identifying and reproducing the cause, then confirming the crash no longer occurs.

Written by the indexing model from the issue text.