vppinfra: do not crash if format string is null

vppinfra: do not crash if format string is null

Sending 128 bytes of zeroes via API shared memory interface (or putting any other message ID that results in
null message name - e.g. 2,4,5) results in this crash:

Program received signal SIGSEGV, Segmentation fault.
va_format (s=0x7fffb5643a60 "memclnt_create", fmt=0x0, va=0x7fffa739ccf0) at /home/ubuntu/vpp/src/vppinfra/format.c:395
395	      c = *f;
(gdb) bt
0  va_format (s=0x7fffb5643a60 "memclnt_create", fmt=0x0, va=0x7fffa739ccf0) at /home/ubuntu/vpp/src/vppinfra/format.c:395
1  0x00007ffff50e7259 in elog_string (em=0x7ffff5f2fbc8 <vlib_global_main+968>, fmt=0x0) at /home/ubuntu/vpp/src/vppinfra/elog.c:592
2  0x00007ffff7bc4cec in vl_msg_api_handler_with_vm_node (am=0x7ffff7dd1c90 <api_global_main>, vlib_rp=0x130023000, the_msg=0x130086d50, vm=0x7ffff5f2f800 <vlib_global_main>,
   node=0x7fffb5edd4c0, is_private=0 '\000') at /home/ubuntu/vpp/src/vlibapi/api_shared.c:585
3  0x00007ffff7ba6c86 in void_mem_api_handle_msg_i (am=0x7ffff7dd1c90 <api_global_main>, vlib_rp=0x130023000, vm=<optimized out>, node=<optimized out>, is_private=0 '\000')
   at /home/ubuntu/vpp/src/vlibmemory/memory_api.c:696
4  vl_mem_api_handle_msg_main (vm=0x7ffff5f2f800 <vlib_global_main>, node=0x7fffb5edd4c0) at /home/ubuntu/vpp/src/vlibmemory/memory_api.c:707
5  0x00007ffff7bb573e in vl_api_clnt_process (vm=0x7ffff5f2f800 <vlib_global_main>, node=0x7fffb5edd4c0, f=<optimized out>) at /home/ubuntu/vpp/src/vlibmemory/vlib_api.c:338
6  0x00007ffff5cb3bb7 in vlib_process_bootstrap (_a=<optimized out>) at /home/ubuntu/vpp/src/vlib/main.c:1477
7  0x00007ffff514088c in clib_calljmp () from /home/ubuntu/vpp/build-root/install-vpp-native/vpp/lib/libvppinfra.so.21.01
8  0x00007fffa95b0550 in ?? ()
9  0x00007ffff5ca93e2 in vlib_process_startup (vm=0x7ffff5f2f800 <vlib_global_main>, p=0x7fffb5edd4c0, f=0x0) at /home/ubuntu/vpp/src/vlib/main.c:1502
10 dispatch_process (vm=0x7ffff5f2f800 <vlib_global_main>, p=0x7fffb5edd4c0, f=0x0, last_time_stamp=<optimized out>) at /home/ubuntu/vpp/src/vlib/main.c:1558
11 0x0000000000000000 in ?? ()
(gdb)

I am not sure if this place is better to fix it or the 4-5 places in vl_msg_api_handler_with_vm_node that it gets called from,
but submitting this one since it is the shortest

Type: fix
Change-Id: I659d2bea7405d8763181336f35ef468682f64cf2
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>