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Correct and enable the range check on statepos when loading

statepos == 0 shouldn't ever occur in a save file because it indicates
an uninitialised midend.  OTOH statepos == nstates is normal.  Also
added an equivalent assertion when saving because Simon and I spent
some time discussing whether it could happen.

--- a/midend.c

+++ b/midend.c

@@ -2091,6 +2091,7 @@

         char buf[80];

         sprintf(buf, "%d", me->nstates);

         wr("NSTATES", buf);

+        assert(me->statepos >= 1 && me->statepos <= me->nstates);

         sprintf(buf, "%d", me->statepos);

         wr("STATEPOS", buf);

     }

@@ -2345,8 +2346,9 @@

         ret = "Game private description in save file is invalid";

         goto cleanup;

     }

-    if (data.statepos < 0 || data.statepos >= data.nstates) {

+    if (data.statepos < 1 || data.statepos > data.nstates) {

         ret = "Game position in save file is out of range";

+        goto cleanup;

     }

     if (!data.states) {