Avoid NPE if database doesn't match expected vendor

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@675911 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/SQLExec.java

@@ -436,6 +436,11 @@ public class SQLExec extends JDBCTask {
             t.setSrc(srcFile);
             t.addText(sqlCommand);
 
+            if (getConnection() == null) {
+                // not a valid rdbms
+                return;
+            }
+
             try {
                 PrintStream out = System.out;
                 try {