Apache NuttX RTOS: tools/bdf-converter.: tools/bdf-converter: Fix loop termination condition.

Out-of-bounds Write resulting in possible Heap-based Buffer Overflow vulnerability was discovered in tools/bdf-converter font conversion utility that is part of Apache NuttX RTOS repository. This standalone program is optional and neither part of NuttX RTOS nor Applications runtime, but active bdf-converter users may be affected when this tool is exposed to external provided user data data (i.e. publicly available automation). This issue affects Apache NuttX: from 6.9 before 12.9.0. Users are recommended to upgrade to version 12.9.0, which fixes the issue.