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Correct a range check in Magnets' layout verification

Squares in the grid are numbered from 0, so the upper limit check
needs to use "<=" rather than "<".  Without this, invalid descriptions
can cause a read overrun off the end of the board.

--- a/magnets.c

+++ b/magnets.c

@@ -520,7 +520,7 @@

      * (i.e. each end points to the other) */

     for (idx = 0; idx < state->wh; idx++) {

         if (state->common->dominoes[idx] < 0 ||

-            state->common->dominoes[idx] > state->wh ||

+            state->common->dominoes[idx] >= state->wh ||

             state->common->dominoes[state->common->dominoes[idx]] != idx) {

             *prob = "Domino descriptions inconsistent";

             goto done;