Add ubsan to the asan CI check.

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

.cirrus.yml

@@ -73,9 +73,12 @@ gcc_task:
        USE_CONFIG: no
     - environment:
        USE_CONFIG: yes
-       CFLAGS: -std=c99 -pedantic -Wformat=2 -fsanitize=address
-       LDFLAGS: -fsanitize=address
+       # With gcc13 -O2 -Wformat=2 -fsanitize=undefined jointly produce a
+       # false positive in hts.c, disabled with -Wformat-truncation=0.
+       CFLAGS: -std=c99 -pedantic -Wformat=2 -Wformat-truncation=0 -fsanitize=address,undefined
+       LDFLAGS: -fsanitize=address,undefined
        USE_LIBDEFLATE: yes
+       UBSAN_OPTIONS: print_stacktrace=1:halt_on_error=1
 
   install_script: |
     apt-get update