Fix tab-completion coverity issue

Change-Id: I051d015e7eee621dbef273b2c57449ea4c44b768
Signed-off-by: Yoann Desmouceaux <ydesmouc@cisco.com>
diff --git a/src/vlib/cli.c b/src/vlib/cli.c
index 9e14bee..48cf042 100644
--- a/src/vlib/cli.c
+++ b/src/vlib/cli.c
@@ -333,7 +333,8 @@
   clib_bitmap_free (match_bitmap);
   unformat_free (&input);
 
-  vec_sort_with_function (result, vlib_cli_cmp_strings);
+  if (result)
+    vec_sort_with_function (result, vlib_cli_cmp_strings);
   return result;
 }