Do not try to 'helpfully' fixup the length of the data provided by the
client when FILECONTENTS_SIZE is present. This can lead to a crash if
the client wants to report an error in msgFlags, sets cbRequested to
zero, and does not provide any data in requestedData. For example,
XFreeRDP does this in xf_cliprdr_send_file_contents_failure() and
xf_cliprdr_clipboard_file_size_failure().