added a check to the open function to prevent losing a reference to an open database connection

Former-commit-id: 62e8386dce392d8c7fbc58febc4f5d4bc35e84f0

dependency-check-core/src/main/java/org/owasp/dependencycheck/data/nvdcve/CveDB.java

@@ -87,7 +87,9 @@ public class CveDB {
      * @throws DatabaseException thrown if there is an error opening the database connection
      */
     public final void open() throws DatabaseException {
-        conn = ConnectionFactory.getConnection();
+        if (!isOpen()) {
+            conn = ConnectionFactory.getConnection();
+        }
     }
 
     /**