Fix error that exceed Clang's default -fconstexpr-steps value

[gnaggnoyil]

From comments of the new test case added in NanoLogCpp17test.cc:

// This test is mainly intended to ensure that the constexpr implementation
// of getNumNibblesNeeded shall not exceed the maximum constexpr evaluation
// steps of a compiler. Implementation as of commit 2a94d70 would be
// evaluated in 1395813 steps under Clang 13.0.1-rc1, which is greater than
// the default value of Clang's `-fconstexpr-steps`, 1048576.