Fix SUBSTR() so that it returns NULL if any argument is NULL.

Ticket #3626. (CVS 6228)

FossilOrigin-Name: 779fce82b7a89b972da488ce8bd35e23ca120c04

src/func.c

@@ -16,7 +16,7 @@
 ** sqliteRegisterBuildinFunctions() found at the bottom of the file.
 ** All other code has file scope.
 **
-** $Id: func.c,v 1.213 2009/02/01 19:23:32 drh Exp $
+** $Id: func.c,v 1.214 2009/02/01 19:42:38 drh Exp $
 */
 #include "sqliteInt.h"
 #include <stdlib.h>
@@ -171,6 +171,11 @@ static void substrFunc(
   i64 p1, p2;
 
   assert( argc==3 || argc==2 );
+  if( sqlite3_value_type(argv[1])==SQLITE_NULL
+   || (argc==3 && sqlite3_value_type(argv[2])==SQLITE_NULL)
+  ){
+    return;
+  }
   p0type = sqlite3_value_type(argv[0]);
   if( p0type==SQLITE_BLOB ){
     len = sqlite3_value_bytes(argv[0]);