ODD creation: Return null instead of exception on overflow

After the previous changes, we are now ready to do MTBDD model checking
even if we can't create the ODD (number of reachable states too large to fit
into an int64_t).

prism/src/odd/ODDUtils.java

@@ -67,7 +67,7 @@ public class ODDUtils
 	private static native long ODD_BuildODD(long dd, long vars, int num_vars);
 	/**
 	 *  Build an ODD.
-	 * @throws PrismException if the ODD could not be constructed
+	 *  If the ODD could not be constructed, returns {@code null}.
 	 */
 	public static ODDNode BuildODD(JDDNode dd, JDDVars vars) throws PrismException
 	{
@@ -78,7 +78,9 @@ public class ODDUtils
 
 		long res = ODD_BuildODD(dd.ptr(), vars.array(), vars.n());
 		if (res == 0) {
-			throw new PrismException("Can not construct ODD for this model, number of states too large: " + JDD.GetNumMintermsString(dd, vars.n()) + " states");
+			// we could not build the ODD (i.e., we had more than Long.MAX_LONG states
+			// we return null and will have to live with the limited functionality
+			return null;
 		}
 		return new ODDNode(res);
 	}