operations: fix cat --offset before the start of a file printing its end

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When a negative --offset reached back past the start of a file and
--count was set, rclone cat printed bytes from the end of the file
instead of from its start. For example on a 10 byte file

    rclone cat --offset -12 --count 5 file

printed the last 2 bytes rather than the first 5. With several files
of different sizes, each file shorter than -offset was affected.

The start of the range stayed negative after the file size was added,
and a negative Start with an End set is a suffix range (the last End
bytes). Clamp the start to 0 instead, which matches what
`tail -c 12 file | head -c 5` prints. --tail and --offset without
--count were not affected.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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