This is a patch to fix my previous report of a regression in capture
behavior between 9.3 and 9.3.6:

> Basically, the last position in the narrowed org-capture is actually
> the first position on the next line, so when you go to (end-of-buffer)
> and start typing you start clobbering the next headline.

The fix already landed in cb2774d1a is inadequate for me as the subtree
structure can still be broken during the capture process.  I think this
is the more correct approach, though I haven't done much testing outside
of my own workflow and `make test`.  It seems to be the same behavior as
9.3.