.travis.yml: add clang ubsan job

We document this on our wiki and we might as well catch it in our CI
rather than waiting for it to be picked up on merge:

  https://wiki.qemu.org/Testing#clang_UBSan

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>

.travis.yml

@@ -152,6 +152,13 @@ matrix:
       compiler: clang
 
 
+    - env:
+        - CONFIG="--target-list=${MAIN_SOFTMMU_TARGETS} "
+      compiler: clang
+      before_script:
+        - ./configure ${CONFIG} --extra-cflags="-fsanitize=undefined -Werror" || { cat config.log && exit 1; }
+
+
     - env:
         - CONFIG="--disable-user --target-list-exclude=${MAIN_SOFTMMU_TARGETS}"
       compiler: clang