Added null-check in release

Author: JanHolger

src/main/java/org/javawebstack/orm/connection/pool/SQLPool.java

@@ -42,6 +42,9 @@ public PooledSQL get() {
     }
 
     public void release(SQL sql) {
+        if(sql == null) {
+            throw new IllegalArgumentException("sql must not be null");
+        }
         if(!closing) {
             if(sql instanceof PooledSQL) {
                 sql.close();