Simple test script, raising the segfault:

#include <editline/readline.h>
int main (int argc, char *argv[]) {
  clear_history();
  return 0;
}

Of course using_history() should be called first.

On PHP side, calling using_history during module init is not perfect as it raise some issue: https://bugs.php.net/65714

Studying libedit code, each *_history function start with a check (eq to using_history function)

        if (h == NULL || e == NULL)
                rl_initialize();

The clear_history is the only function where this check is missing.
Of course, this have be workaround in php, https://bugs.php.net/66412

I think it will be more secure to add this check.