Improve HostDeviceVector exception safety (#4301)

* make the assignments of HostDeviceVector exception safe.
* storing a dummy GPUDistribution instance in HDV for CPU based code.
* change testxgboost binary location to build directory.

tests/travis/run_test.sh

@@ -123,8 +123,8 @@ if [ ${TASK} == "cmake_test" ]; then
     PLUGINS="-DPLUGIN_LZ4=ON -DPLUGIN_DENSE_PARSER=ON"
     cmake .. -DGOOGLE_TEST=ON -DGTEST_ROOT=$PWD/../gtest/ ${PLUGINS}
     make
-    cd ..
     ./testxgboost
+    cd ..
     rm -rf build
 fi
 
@@ -170,10 +170,10 @@ if [ ${TASK} == "sanitizer_test" ]; then
       -DCMAKE_CXX_FLAGS="-fuse-ld=gold" \
       -DCMAKE_C_FLAGS="-fuse-ld=gold"
     make
-    cd ..
 
     export ASAN_SYMBOLIZER_PATH=$(which llvm-symbolizer)
     ASAN_OPTIONS=symbolize=1 ./testxgboost
+    cd ..
     rm -rf build
     exit 0
 fi