fixed compiler warning

sql/item_strfunc.h

@@ -695,7 +695,7 @@ public:
   void fix_length_and_dec()
   {
     ulonglong max_result_length= (ulonglong) args[0]->max_length * 2 + 2;
-    max_length= min(max_result_length, MAX_BLOB_WIDTH);
+    max_length= (uint32) min(max_result_length, MAX_BLOB_WIDTH);
     collation.set(args[0]->collation);
   }
 };