Update php-apcu to 5.1.10 - small bugfixes

Changes since 5.1.9:

- fix gh#247 when a NUL char is used as key, apcu_fetch(array) truncates the key
- fix gh#248 apcu_fetch may return values causing zend_mm_corruption or segfaults
when custom serializer is used
- fix gh#260 apcu.serializer=default results in segfault
- fix gh#274 non-portable shell == in config.m4
- fix crash when passing bad array to apcu_delete
- improve fix gh#266 refcounting errors in APCIterator
- fix for PHP 7.3 compatibility

(jdolecek)