Fix local variable 'upload' referenced before assignment in Remote.upload_chunks()

MemoryError: 
  File "nxdrive\client\remote_client.py", line 433, in upload_chunks
  File "nxdrive\engine\dao\sqlite.py", line 2017, in get_upload
  File "nxdrive\engine\dao\sqlite.py", line 1979, in get_uploads
  File "nxdrive\engine\dao\sqlite.py", line 1979, in <listcomp>

UnboundLocalError: local variable 'upload' referenced before assignment
  File "nxdrive\engine\processor.py", line 270, in _execute
  File "nxdrive\engine\processor.py", line 779, in _synchronize_locally_created
  File "nxdrive\client\remote_client.py", line 709, in stream_file
  File "nxdrive\client\remote_client.py", line 401, in upload
  File "nxdrive\client\remote_client.py", line 550, in upload_chunks