Fix backtracking for arity-2 predicates

In commit d53fd3122110ad5440de6b8a4edfe65a17d6f457 there
was an accidental removal restoring the continuation when
a /2 predicate backtracked to try another choice point.

Put back the missing "engine.cont = this.cont" register.

Change-Id: I244c3e2564cff04fef51afdcecb413617f7adae8

src/lang/ChoicePointFrame.java

diff --git a/src/lang/ChoicePointFrame.java b/src/lang/ChoicePointFrame.java
index 8d66621..e7b6b5c 100644
--- a/src/lang/ChoicePointFrame.java
+++ b/src/lang/ChoicePointFrame.java
@@ -51,6 +51,7 @@
       }
 
       void restore(Prolog engine) {
+        engine.cont = this.cont;
         engine.r1 = this.r1;
         engine.r2 = this.r2;
       }