Incorrect memory shortage warning when memory_limit = unlimited

The status report incorrectly warns about a memory shortage when the setting memory_limit in php.ini is set to -1 (unlimited memory).

Steps to reproduce:

1. Set memory_limit in php.ini to -1 (see http://be.php.net/ini.core#ini.memory-limit for info)
2. Visit the status report: there will be a warning for "PHP memory limit".

Patch attached fixes this.

related issue: #309457: Allow profiles to specify required memory in .info file