Any time `SFDDumpUndo()` or `SFDGetUndo()` are run, they end up calling
`SplineCharCopy()` with `NULL` for the `SplineFont *into` parameter.
When this happens, `SplineCharCopy()` attempts to copy the anchors into
the new SplineChar's `parent` font, but since that is set to `NULL` it
crashes when `AnchorPointsDuplicate()` attempts to read from
`sc->parent->anchor`.

Because `SFDDumpUndo()` and `SFDGetUndo()` appear to only do this so
that they can have a copy of the character's hints at a given undo state
(and then copy thoes hints into or out of the .sfd file), never affect
anchor classes, and delete the parentless SplineChar immediately after,
I've determined that it's safe to simply not duplicate the AnchorPoints
when `into` is set to `NULL`.

To do this, instead of setting `nsc->anchor` to the output returned by
`AnchorPointsDuplicate()`, I use a ternary statement to detect whether
or not `into` is NULL. If it is, then `nsc->anchor` is set to NULL, and
otherwise the value returned by `AnchorPointsDuplicate()` is used. I
decided not to do the reverse (which could have made the code shorter)
simply because line 505 already includes a similar ternary statement
that checks if `into` is `NULL`, and I decided to use the same format.

This fixes fontforge#5130, and is a slightly more elegant variation of the same
fix that I had proposed back then (more elegant because I skip the
entire `AnchorPointsDuplicate()` function call instead of merely
skipping over the loop that makes up the majority of the function). I
don't see a reason for this to be functionally any different from the
originally proposed solution, and I've extensively used FontForge with
that change made with no further `AnchorPoint` crashes.

A more 'proper' fix would probably include systematic changes to how
hint undoes are saved and loaded from SFD files, but that's beyond my
skill and, quite honestly, not something I even care about that much.
That code appears to work despite it's oddities, and simply checking if
`into==NULL` before doing anything with the `AnchorPoints` allows all
the hint-related stuff that `SplineCharCopy()` does continue to work for
the sake of both `SFDDumpUndo()` and `SFDGetUndo()`.