When asking for the last N bytes of a file, and that size was larger
than the file size, it would miss the first byte due to a logic error.
The fixed range parser is now made a common function in the file now
renamed to vssh.c (from curl_path.c) - used by both backends.
Unit test 2605 verifies the parser.
Reported-by: Stanislav Fort (Aisle Research)
Closes#19460
