odp-util: Fix abort while formatting nsh actions.

OVS should not exit if it cannot format NSH actions for the user.
It should just report the error like the other formatting functions do.

Credit to OSS-Fuzz.

Reported-at: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=21509
Fixes: 1fc11c5 ("Generic encap and decap support for NSH")
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Mark Gray <mark.d.gray@redhat.com>

lib/odp-util.c

@@ -391,7 +391,8 @@ format_odp_push_nsh_action(struct ds *ds,
         break;
     }
     default:
-        OVS_NOT_REACHED();
+        ds_put_cstr(ds, ",<error: unknown mdtype>");
+        break;
     }
     ds_put_format(ds, ")");
 }