more parsing errors found by TestRandomSQL

h2/src/main/org/h2/command/Parser.java

@@ -2626,6 +2626,9 @@ public class Parser {
             read("OVER");
             read("(");
             read(")");
+            if (currentSelect == null && currentPrepared == null) {
+                throw getSyntaxError();
+            }
             return new Rownum(currentSelect == null ? currentPrepared
                     : currentSelect);
         default:
@@ -2963,6 +2966,9 @@ public class Parser {
             if (readIf("(")) {
                 read(")");
             }
+            if (currentSelect == null && currentPrepared == null) {
+                throw getSyntaxError();
+            }
             r = new Rownum(currentSelect == null ? currentPrepared
                     : currentSelect);
             break;

h2/src/main/org/h2/expression/Rownum.java

@@ -21,6 +21,9 @@ public class Rownum extends Expression {
     private final Prepared prepared;
 
     public Rownum(Prepared prepared) {
+        if (prepared == null) {
+            throw DbException.throwInternalError();
+        }
         this.prepared = prepared;
     }