0005083: setup impossible with memory_limit = -1

This fixes the improper validation of memory_limit.
For further information see https://bugs.oxid-esales.com/view.php?id=5083

To be able to add an unit test for this case I had to change the method signature of the method `oxSysRequirements::checkMemoryLimit()`. 

Since this bug is very annoying and often happens to appear in development environments while modules for this method should be very rare I committed this to the current maintenance branch.

This is a bug fix ;)