[libxml2] Make Travis print UBSan stacktraces

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 863b57925a5a2cd45a12ceb935dd82a411ab746e
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Thu Jun 1 17:53:16 2017 +0200

    Make Travis print UBSan stacktraces

 .travis.yml |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/.travis.yml b/.travis.yml
index 36f5048..5975976 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -17,6 +17,7 @@ matrix:
       dist: trusty
       env: CONFIG="--without-python"
            CFLAGS="-O2 -g -fno-omit-frame-pointer -fsanitize=address,undefined -fno-sanitize-recover"
+           UBSAN_OPTIONS=print_stacktrace=1
 script: sh autogen.sh $CONFIG && make -j2 V=1 && make check
 git:
   depth: 10