[libcalamares] Avoid nullptr crashes

- The Python testmodule script can end up calling in to System
   methods (via System::instance()). This is unusual, and the
   System instance has not been created at that point.
   Now, create an instance and warn about it.

src/libcalamares/utils/CalamaresUtilsSystem.cpp

@@ -58,8 +58,15 @@ System::~System()
 {}
 
 
-System*System::instance()
+System*
+System::instance()
 {
+    if ( !s_instance )
+    {
+        cError() << "No Calamares system-object has been created.";
+        cError() << " .. using a bogus instance instead.";
+        return new System( true, nullptr );
+    }
     return s_instance;
 }