Python fixes from José Matos

Python fixes from José Matos

See here:

  https://www.mail-archive.com/search?l=mid&q=a7960223ee91696e916b281d1d9fc55df10a9480.camel%40gmail.com

Description from José pasted below:

  Functions `length_in_bp` and `revert_flex_inset` have an error in a
  (almost) never used path. The reason why this was never triggered is
  because this is part of the code where we try all the best efforts
  to recover from an ill-formed LyX file.

  The problem is that the functions call the `document` variable to
  issue an warning if the file does not follow the usual structure.
  The simple fix is to add that variable (as the first for the
  functions) and to add them at all the places where the functions are
  called.

  For reference this code was introduced by Georg (Baum) more than 8
  years ago and it was never reported. So the issue is not urgent in
  any meaningful way. :-)

  In `lyx2lyx/lyx_2_0.py`, in the function `revert_nameref`, there is
  a call to the document variable. The same as all the above.

  In `lyx2lyx/lyx_2_4.py`, in the `class fontmapping`. Similar to the
  above but now for error.