api: fix trace_plugin_msg_ids segv in trace dump

With 'api-trace { on }' in startup.conf, running 'api trace dump' in
vppctl was causing VPP to seg fault. vl_msg_print_trace() was calling
m->endian_handler() without checking whether its null.

Checking if its non-null prevents a crash, but the trace dump prints
the message IDs for trace_plugin_msg_ids in network byte order. There is
an auto-generated endian function for that message. Set it on the call
to vl_msg_api_config() for trace_plugin_msg_ids so the IDs will be
printed in host byte order in trace dump output.

Type: fix
Fixes: fe45f8f5

Signed-off-by: Matthew Smith <mgsmith@netgate.com>
Change-Id: I0ab463985e9a983155feba13ac4eb99ab883ace6
diff --git a/src/vlibmemory/vlib_api_cli.c b/src/vlibmemory/vlib_api_cli.c
index 5b62e61..f9cda9b 100644
--- a/src/vlibmemory/vlib_api_cli.c
+++ b/src/vlibmemory/vlib_api_cli.c
@@ -683,7 +683,7 @@
       return 0;
     }
 
-  if (clib_arch_is_little_endian)
+  if (clib_arch_is_little_endian && (m->endian_handler != NULL))
     {
       u32 msg_length = vec_len (msg);
       vec_validate (tmpbuf, msg_length - 1);