Kudu scanner : Avoid per slot check of Vector size in KuduScanner::RelocateValuesFromKudu

Status KuduScanner::RelocateValuesFromKudu(Tuple* tuple, MemPool* mem_pool) {
  for (int i = 0; i < string_slots_.size(); ++i) {
    const SlotDescriptor* slot = string_slots_[i];
    // NULL handling was done in KuduRowToImpalaTuple.

CPU Time
1 of 2: 81.2% (6.890s of 8.481s)

impalad ! std::vector<impala::SlotDescriptor*, std::allocator<impala::SlotDescriptor*>>::size - stl_vector.h
impalad ! impala::KuduScanner::KuduRowToImpalaTuple + 0x80 - kudu-scanner.cc:307
impalad ! impala::KuduScanner::DecodeRowsIntoRowBatch + 0xd8 - kudu-scanner.cc:236
impalad ! impala::KuduScanner::GetNext + 0x124 - kudu-scanner.cc:142
impalad ! impala::KuduScanNode::ScannerThread + 0x3fb - kudu-scan-node.cc:429
impalad ! boost::function0<void>::operator() + 0x1a - function_template.hpp:767
impalad ! impala::Thread::SuperviseThread + 0x20c - thread.cc:316
impalad ! operator()<void (*)(const std::basic_string<char>&, const std::basic_string<char>&, boost::function<void()>, impala::Promise<long int>*), boost::_bi::list0> + 0x5a - bind.hpp:457
impalad ! boost::_bi::bind_t<void, void (*)(std::string const&, std::string const&, boost::function<void (void)>, impala::Promise<long>*), boost::_bi::list4<boost::_bi::value<std::string>, boost::_bi::value<std::string>, boost::_bi::value<boost::function<void (void)>>, boost::_bi::value<impala::Promise<long>*>>>::operator() - bind_template.hpp:20
impalad ! boost::detail::thread_data<boost::_bi::bind_t<void, void (*)(std::string const&, std::string const&, boost::function<void (void)>, impala::Promise<long>*), boost::_bi::list4<boost::_bi::value<std::string>, boost::_bi::value<std::string>, boost::_bi::value<boost::function<void (void)>>, boost::_bi::value<impala::Promise<long>*>>>>::run + 0x19 - thread.hpp:116
impalad ! thread_proxy + 0xd9 - [unknown source file]
libpthread.so.0 ! start_thread + 0xd0 - [unknown source file]
libc.so.6 ! clone + 0x6c - [unknown source file]