Fix convert_spaceinset again.

Fix convert_spaceinset again.

The problem here is that
  for i in range(len(document.body)):
sets the range BEFORE we do any of our manipulations. But those manipulations
can make document.body longer. As a result, we can fail to test some lines.
Instead, use:
  while i < len(document.body):
and increment i in the while loop.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@25050 a592a061-630c-0410-9148-cb99ea01b6c8