Fix crash if XDG_CACHE_HOME directory is too small or out of space

Summary:
Incorrect checking for error return code of posix_fallocate() causes function to think that everything is OK, while it is not, causing crash in some cases.

BUG: 400610
CCBUG: 339829

Test Plan:
good test plan provided in https://bugs.kde.org/show_bug.cgi?id=400610 . Works like a charm, tested in KDE Neon dev-ustable

The reason for bug was that return value of posix_fallocate() was assumed to be negative on error, but in fact it is a positive integer. The check was < 0, whi should be != 0. ( http://man7.org/linux/man-pages/man3/posix_fallocate.3.html )

With this fix applied test application does not crash, and the output in console widow is:

No space left on device. Check filesystem free space at your XDG_CACHE_HOME!
The operating system is unable to promise 10547304 bytes for mapped cache, abandoning the cache for crash-safety.
org.kde.kcoreaddons: Failed to establish shared memory mapping, will fallback to private memory -- memory usage will increase

Reviewers: dfaure, Frameworks, mpyne

Reviewed By: dfaure

Subscribers: cfeck, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16744