SshTransport: Fix stat() call on the wrong path.

This prevented download of *any* output file from a remote server.

gc3libs/backends/transport.py

@@ -860,7 +860,7 @@ def get(self, source, destination, ignore_nonexisting=False,
                       ignore_nonexisting, overwrite, changed_only)
 
     def _get_impl(self, source, destination):
-        if self.stat(destination).st_size < self.large_file_threshold:
+        if self.stat(source).st_size < self.large_file_threshold:
             self.sftp.get(source, destination)
         else:
             with self.sftp.open(source) as fsrc: