Very minor issues found with statical analysis.

Very minor issues found with statical analysis.

The Clang Static Analyzer uncovered those issues:

- The variable "changed" in handlers.c is written to, but it's
  never read since that specific write, so the write is not
  necessary.

- In util.c, "tail" may be NULL. In that case, we shouldn't pass
  it to strlen because strlen's behavior is not defined when s is
  NULL.

- In util.c, "write_index" is incremented twice. It's never used
  anymore after being incremented once, so the second increment is
  not necessary.