SFTP: avoid downloading negative sizes!

It is still not clarified exactly why this happens, but libssh2
sometimes report a negative file size for the remote SFTP file and that
deeply confuses libcurl (or crashes it) so this precaution is added to
avoid badness.

Reported by: Ernest Beinrohr
Bug: http://curl.haxx.se/bug/view.cgi?id=3076430