Fix guards of havoc assignments

This was previously taking only the innermost if condition into account
for nested ternary expressions, due to disarding the outer conditions.

Author: thomasspriggs

src/goto-symex/symex_other.cpp

@@ -46,11 +46,11 @@ void goto_symext::havoc_rec(
   {
     const if_exprt &if_expr=to_if_expr(dest);
 
-    guardt guard_t=state.guard;
+    guardt guard_t = guard;
     guard_t.add(if_expr.cond());
     havoc_rec(state, guard_t, if_expr.true_case());
 
-    guardt guard_f=state.guard;
+    guardt guard_f = guard;
     guard_f.add(not_exprt(if_expr.cond()));
     havoc_rec(state, guard_f, if_expr.false_case());
   }