[MCJack123]

Description

This PR fixes an issue with buffer corruption that may occur if a block transfer fails. In FF_GetBuffer, if the underlying block reader in FF_BlockRead partially reads data into the buffer but fails, the function returns a failure state without updating the buffer it attempted reading on. This can cause a future request for the sector previously used for that buffer to return the half-read data for the sector that was being read.

The bug is solved by marking a failed buffer as invalid, and also setting the sector to 0 just to make double sure it doesn't get grabbed again.

Test Steps

1. Call FF_GetBuffer on enough different sectors to saturate the buffer list.
2. Call FF_GetBuffer on a new sector to overwrite an old buffer, but trigger a transfer failure in the middle of reading somehow.
3. Call FF_GetBuffer on the sectors read in step 1 in reverse order until the contents of the buffer do not represent the actual sector contents.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

Related Issue

N/A

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