[Helpers] Fix an invalid memory access.

The constructor of string(char *, index, length) uses strlen() as the following
valgrind result. So we have to add a null terminator.

==31244== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)
==31240== Conditional jump or move depends on uninitialised value(s)
==31240==    at 0x4C2D828: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31240==    by 0x8863A44: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.18)
==31240==    by 0x9F4C99A: SpawnSyncContext::cbData(_GIOChannel*, GIOCondition, void*) (Helpers.cc:129)
==31240==    by 0x5E963A5: g_main_context_dispatch (gmain.c:3065)
==31240==    by 0x5E966F7: g_main_context_iterate.isra.23 (gmain.c:3712)
==31240==    by 0x5E9679B: g_main_context_iteration (gmain.c:3773)
==31240==    by 0x9F486C5: spawnSync(std::string const&, std::string&, std::string&, _GError**) (Helpers.cc:171)
==31240==    by 0x9F487C2: executeCommand(std::string const&) (Helpers.cc:185)
==31240==    by 0x9E9889E: testDBClientHatohol::test_addTriggerInfo() (testDBClientHatohol.cc:389)
==31240==    by 0x4E7C32C: ??? (in /usr/lib/libcutter.so.0.1.0)
==31240==    by 0x4E71C57: ??? (in /usr/lib/libcutter.so.0.1.0)
==31240==    by 0x4E71CD2: cut_test_case_run_with_filter (in /usr/lib/libcutter.so.0.1.0)
==31240==  Uninitialised value was created by a stack allocation
==31240==    at 0x9F4C901: SpawnSyncContext::cbData(_GIOChannel*, GIOCondition, void*) (Helpers.cc:11)