Fix crash in file browser

[sy2002]

Fixing a crash of the file browser that can be provoked when using the "page up" / "page down" (cursor left / right) keys in complex / long / hierarchically deep folder structures, i.e. huge game libraries

[sy2002]

Reproduction (various variants to provoke the bug).

Variant 1:

1. Open the File Browser (OFB) in a directory with less files than the screen allows
2. Right arrow
3. Move upwards using the up arrow until nearly at the top
4. Run/Stop
5. OFB
6. Crash

Variant 2:

1. OFB (like Variant 1)
2. Go to the middle of the screen using arrow down
3. Left arrow
4. Press down for example three times (or more) until you have a subdirectory
5. Enter enter it
6. Go up again using ".."
7. Crash

Variant 3:

1. OFB and navigate to a directory with a lot of entries, for example in a large game library with lots of subdirectories
2. Use the right arrow to go down until you reach the last entry
3. Use the up arrow to go up to page one
4. Enter a subdirectory on page one
5. Use ".."
6. Crash

[sy2002]

Reason for the bug: The absolute index within the linked-list (register R6) was not updated properly. Commit f9b57df fixes this bug.