Merge pull request #134 from k-takata/fix-segv-in-error-str

Fix SEGV in onig_error_code_to_str() (Fix #132)
UNKNOWN_ESC_WARN(ScanEnv *env, int c)
{
  if (onig_warn == onig_null_warn || !RTEST(ruby_verbose)) return ;
  onig_syntax_warn(env, "Unknown escape \\%c is ignored", c);
}

UNKNOWN_ESC_WARN(ScanEnv *env, int c)
{
  if (onig_warn == onig_null_warn || !RTEST(ruby_verbose)) return ;
  onig_syntax_warn(env, "Unknown escape \\%c is ignored", c);
}
