fix 'campos' referenced before assignment

[rvalyi]

sem isso pega erros do tipo:

  File "/odoo/external-src/l10n-brazil/l10n_br_nfe/wizards/import_document.py", line 66, in _onchange_partner_id
    base64.b64decode(self.nfe_xml)
  File "/odoo/external-src/l10n-brazil/l10n_br_nfe/wizards/import_document.py", line 58, in _parse_xml_import_wizard
    document = detectar_chave_edoc(parsed_xml.infNFe.Id[3:])
  File "/usr/local/lib/python3.7/dist-packages/erpbrasil/base/fiscal/edoc.py", line 27, in detectar_chave_edoc
    if not matcher and not campos:
UnboundLocalError: local variable 'campos' referenced before assignment

cc @renatonlima