warning: iteration 5 invokes undefined behavior [-Waggressive-loop-optimizations]

[yuzhimin999]

环境：x86 ubuntu 20.04
报错如下：
[2/3] Building CXX object CMakeFiles/linux_proc_demo.dir/cron.cpp.o
/home/embedded/CLionProjects/linux_proc_demo/cron.cpp: In member function ‘cronTab::byte cronTab::cron::index(cronTab::field_name)’:
/home/embedded/CLionProjects/linux_proc_demo/cron.cpp:32:18: warning: iteration 5 invokes undefined behavior [-Waggressive-loop-optimizations]
index[i] = index[i-1] + field_size[i-1];
~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/embedded/CLionProjects/linux_proc_demo/cron.cpp:31:23: note: within this loop
for(byte i(1); i<=field_name::year; i++)
如果在Jenkins上会报错，编译不通过
In member function ‘cronTab::byte cronTab::cron::index(cronTab::field_name)’:
error: iteration 5u invokes undefined behavior [-Werror=aggressive-loop-optimizations]
index[i] = index[i-1] + field_size[i-1];
^
note: containing loop
for(byte i(1); i<=field_name::year; i++)
^

In member function ‘cronTab::cron& cronTab::cron::assign(std::__cxx11::string)’:
error: array subscript is above array bounds [-Werror=array-bounds]
}return index[nfield];