Fix crash when using QLockFile in a global destructor

(for instance any global object which writes out to a config file in the
destructor). If the global cache isn't available anymore, don't use it.

Change-Id: I851a6e394d0b073aebf3ffd88b1966d424bfb92e
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/io/qlockfile_unix.cpp

@@ -136,6 +136,8 @@ static QBasicMutex fcntlLock;
 static bool fcntlWorksAfterFlock(const QString &fn)
 {
     QMutexLocker lock(&fcntlLock);
+    if (fcntlOK.isDestroyed())
+        return QLockFilePrivate::checkFcntlWorksAfterFlock(fn);
     bool *worksPtr = fcntlOK->object(fn);
     if (!worksPtr) {
         worksPtr = new bool(QLockFilePrivate::checkFcntlWorksAfterFlock(fn));

tests/auto/corelib/io/qlockfile/tst_qlockfile.cpp

@@ -546,5 +546,15 @@ bool tst_QLockFile::overwritePidInLockFile(const QString &filePath, qint64 pid)
     return f.write(buf) == buf.size();
 }
 
+struct LockFileUsageInGlobalDtor
+{
+    ~LockFileUsageInGlobalDtor() {
+        QLockFile lockFile(QDir::currentPath() + "/lastlock");
+        QVERIFY(lockFile.lock());
+        QVERIFY(lockFile.isLocked());
+    }
+};
+LockFileUsageInGlobalDtor s_instance;
+
 QTEST_MAIN(tst_QLockFile)
 #include "tst_qlockfile.moc"