Description:
------------
If mbstring is built without mbregex support,
mbstring.http_output_conv_mimetypes is basically ignored.

The problem is that the return value of the PCRE fallback
implementation of _php_mb_match_regex()[1] is not “normalized” to
return a truthy value in case of match, and a falsey value
otherwise.

[1] <https://github.com/php/php-src/blob/php-7.3.0beta1/ext/mbstring/mbstring.c#L1071-L1074>

Test script:
---------------
ext/mbstring/tests/mb_output_handler_pattern-04.phpt

Expected result:
----------------
Test succeeds

Actual result:
--------------
Test fails