acd: seed bestPerm to avoid an uninitialised read in enumerate_iset_combinations

bestPerm is only written inside the 'cost < best_cost' branch. When no
combination beats the initial best_cost -- which happens for an infeasible
free-set size -- the array is never written, yet the tail of the function still
evaluates permutations[bestPerm[i]]. That reads uninitialised stack and then
uses the value to index permutations[], so it is an out-of-bounds read as well.

Upstream results are unaffected in practice because the caller discards the
permutation on that path, but it is undefined behaviour and it becomes a hard
segfault as soon as the stack layout changes -- adding two members to the
decomposer object was enough to trigger it reliably.

Seeding the identity permutation in the existing initialisation loop is
sufficient and costs nothing.

src/map/if/acd/ac_decomposition.hpp

@@ -482,6 +482,11 @@ private:
     for ( uint32_t i = 0; i < num_vars; ++i )
     {
       pComb[i] = pInvPerm[i] = i;
+      /* bestPerm is written only when some combination beats the initial
+       * best_cost.  When none does, the loop below still evaluates
+       * permutations[bestPerm[i]], which reads uninitialised stack and then
+       * indexes permutations[] with it.  Seed the identity permutation. */
+      bestPerm[i] = i;
     }
 
     /* early bail-out conditions */