api: fix trace_plugin_msg_ids segv in trace dump

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