Add ubsan to the asan CI check.

We already have address sanitizer, so add "undefined" into the mix.

.cirrus.yml

@@ -109,8 +109,9 @@ ubuntu_task:
        DO_UNTRACKED_FILE_CHECK: yes
     - environment:
        USE_CONFIG: yes
-       CFLAGS: -g -Wall -O3
+       CFLAGS: -g -Wall -O3 -fsanitize=undefined
        USE_LIBDEFLATE: yes
+       UBSAN_OPTIONS: print_stacktrace=1:halt_on_error=1
 
   # NB: we could consider building a docker image with these
   # preinstalled and specifying that instead, to speed up testing.