* PHP-7.3:
  Add SKIPIF to test requiring mbregex

Ideally "c" would be an unsigned integer...

I replaced it with a multiplication overflow check in
18599f9c52959b2e8cbfac57e278644499a3547d. However, we need both,
because the code for restoring the number can't handle numbers
with many leading zeros right now and I don't feel like teaching it.

* PHP-7.3:
  Fix mb_ord() crash if internal encoding not supported

enc_name can be NULL here. Take the name from the mbfl_encoding
instead.

Check for multiplication overflow rather than number of digits.

* PHP-7.3:
  Reset MBREX(search_re) in RSHUTDOWN

This is going to cause a segfault if reused in the next request.
To illustrate the issue, run these two scripts in sequence with
the built-in server:

// script1.php
mb_ereg_search_init('foobar');
mb_ereg_search('foo');

// script2.php
var_dump(mb_ereg_search_init("foobar"));
var_dump(mb_ereg_search_pos());

* PHP-7.3:
  Fix use of mb_ereg_search_getregs() after invalid pattern