BIER CLI show commands; no crash on non-existant objects (VPP-1303)

DBGvpp# sh bier disp entry
DBGvpp# sh bier disp entry 0
No such BIER disp entry: 0
DBGvpp# sh bier disp table
DBGvpp# sh bier disp table 0
No such BIER disp table: 0
DBGvpp# sh bier disp table 11
No such BIER disp table: 11
DBGvpp#

DBGvpp# sh bier bift
no BIFT entries
DBGvpp# sh bier bift set 0
no BIFT entries
DBGvpp# sh bier bift set 0 sd 0 bsl 0
no BIFT entries
DBGvpp#

DBGvpp# sh bier fib
No BIER tables
DBGvpp# sh bier fib 0
DBGvpp# sh bier fib 0 4

DBGvpp# sh bier fmask
DBGvpp# sh bier fmask 2
No BIER f-mask 2

DBGvpp# sh bier imp
DBGvpp# sh bier imp 0
No such BIER imposition: 0

Change-Id: Ibadac3441dd8a6d1b96bd9ee4358e28498875b95
Signed-off-by: Neale Ranns <nranns@cisco.com>
diff --git a/src/vnet/bier/bier_disp_table.c b/src/vnet/bier/bier_disp_table.c
index c663286..68edb08 100644
--- a/src/vnet/bier/bier_disp_table.c
+++ b/src/vnet/bier/bier_disp_table.c
@@ -385,8 +385,15 @@
     }
     else
     {
-        vlib_cli_output(vm, "%U", format_bier_disp_table, bdti, 0,
-                        BIER_SHOW_DETAIL);
+        if (pool_is_free_index(bier_disp_table_pool, bdti))
+        {
+            vlib_cli_output(vm, "No such BIER disp table: %d", bdti);
+        }
+        else
+        {
+            vlib_cli_output(vm, "%U", format_bier_disp_table, bdti, 0,
+                            BIER_SHOW_DETAIL);
+        }
     }
     return (NULL);
 }