[truetype] Reset localpoints when varying cvt.

When iterating over the cvt tuples and reading in the points it is necessary
to set all of `localpoints`, `points`, and `point_count` in all cases.  The
existing code did not reset `localpoints` to `NULL` when there were no
private point numbers.  If the previous tuple did have private point numbers
and set `localpoints` to `ALL_POINTS` this would not be cleared and the
wrong branch would be taken later, leading to possible heap buffer overflow.

* src/truetype/ttgxvar.c (tt_face_vary_cvt): Reset `localpoints` to `NULL`
when it isn't valid.

Fixes: https://crbug.com/1284742

src/truetype/ttgxvar.c

@@ -3476,6 +3476,7 @@
       }
       else
       {
+        localpoints = NULL;
         points      = sharedpoints;
         point_count = spoint_count;
       }