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grid_edge_bydots_cmpfn: remove dangerous pointer comparison.

Commit e6cdd70df867f06 made the grid_dot structures for a grid no
longer be elements of the same array. But I didn't notice that
grid_edge_bydots_cmpfn was doing pointer subtraction on them on the
assumption that they were.

Fixed by comparing the dots' new index fields, which should correspond
exactly to their previous positions in the single array, so the
behaviour should be just what it was before the change.

--- a/grid.c

+++ b/grid.c

@@ -342,21 +342,23 @@

     grid_edge *b = v2;

     grid_dot *da, *db;

-    /* Pointer subtraction is valid here, because all dots point into the

-     * same dot-list (g->dots).

-     * Edges are not "normalised" - the 2 dots could be stored in any order,

+    /* Edges are not "normalised" - the 2 dots could be stored in any order,

      * so we need to take this into account when comparing edges. */

     /* Compare first dots */

     da = (a->dot1 < a->dot2) ? a->dot1 : a->dot2;

     db = (b->dot1 < b->dot2) ? b->dot1 : b->dot2;

-    if (da != db)

-        return db - da;

+    if (da->index < db->index)

+        return -1;

+    if (da->index > db->index)

+        return +1;

     /* Compare last dots */

     da = (a->dot1 < a->dot2) ? a->dot2 : a->dot1;

     db = (b->dot1 < b->dot2) ? b->dot2 : b->dot1;

-    if (da != db)

-        return db - da;

+    if (da->index < db->index)

+        return -1;

+    if (da->index > db->index)

+        return +1;

     return 0;

 }