Prevent NPE in case the CveDB.getInstance() failed. This NPE masked the actual cause thereby hampering issue analysis

dependency-check-core/src/main/java/org/owasp/dependencycheck/data/update/NvdCveUpdater.java

@@ -172,7 +172,9 @@ public class NvdCveUpdater implements CachedWebDataSource {
             throw new UpdateException("Database Exception", ex);
         } finally {
             shutdownExecutorServices();
-            cveDb.close();
+            if(cveDb != null) {
+                cveDb.close();
+            }
             if (lock != null) {
                 try {
                     lock.release();