Fix bad index in 'zdd_disjoint' path pruning

This mistake was introduced in the refactoring of 18c3619 . As such, it does not
produce incorrect answers. But, it does make it slower that it truly needs to
be.

src/adiar/zdd/pred.cpp

@@ -237,7 +237,7 @@ namespace adiar
       }
 
       // Has the right-hand side fallen out of its set?
-      if (rp[0].is_false()) {
+      if (rp[1].is_false()) {
         return false;
       }