One important point in the cliprdr protocol is that the peers are not
allowed to request file sizes and ranges if the clipboard content
changes. File locking should be used to gain this ability. However, our
file list is still accessible after new data is set into wClipboard.
Catch this error by storing the sequence number of the file list when it
is set and checking that it is still in effect at the time when the
client requests file sizes or ranges. There is a small chance of false
positives when the sequence number overflows, but I guess we can safely
ignore it.
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