ftrace: Avoid potential division by zero in function_stat_show()

In the Linux kernel, the following vulnerability has been resolved:

ftrace: Avoid potential division by zero in function_stat_show()

Check whether denominator expression x * (x - 1) * 1000 mod {2^32, 2^64}

produce zero and skip stddev computation in that case.

For now don't care about rec->counter * rec->counter overflow because

rec->time * rec->time overflow will likely happen earlier.