CI: use address sanitizer

* .travis.yml (matrix): Use the latest (available) clang with asan.

.travis.yml

@@ -78,6 +78,16 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-5.0 && CXX=clang++-5.0 && CPPFLAGS=-DNDEBUG && CFLAGS=-O3 && CXXFLAGS=-O3"
 
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - llvm-toolchain-trusty-5.0
+          packages:
+            - clang-5.0
+      env:
+        - MATRIX_EVAL="CC='clang-5.0 -fsanitize=address' && CXX='clang++-5.0 -fsanitize=address'"
+
     - os: linux
       addons:
         apt: