Fix possible null dereference in mtom caching sample
diff --git a/samples/mtom_caching_callback/mtom_caching_callback.c b/samples/mtom_caching_callback/mtom_caching_callback.c
index 2a4ff01..411aeaf 100644
--- a/samples/mtom_caching_callback/mtom_caching_callback.c
+++ b/samples/mtom_caching_callback/mtom_caching_callback.c
@@ -78,7 +78,7 @@
     }
 
     #if !defined(WIN32)
-    {
+    if (file_name) {
         int ret = 0;
         axis2_char_t permission_str[256];
         sprintf(permission_str, "chmod 777 %s", file_name);