Fix division by zero for low frequency timers for CV statistics

[ivafanas]

clock_gettime resolution is implementation defined:
https://linux.die.net/man/3/clock_gettime

double StatisticsCV(const std::vector<double>& v) function gets vector of zeroes on system with low frequency timers in user_counters_tabular_test test BM_Counters_Tabular/repeats:2/threads:1_cv case. Division result is nan and test fails due to regexp mismatch.

Additionally, division by zero (even FP zero) is a UB in C++ and should be avoided.

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[dmah42]

nice :) thanks.