Video: prevent unlikely crash during recording

This can happen when user enables a telemetry fact and then that fact is
removed from codebase sometime later. SubtitleWriter will load NULL fact
from user configuration and then try to dereference.

Stumbled upon this while playing around with custom build. Don't think
this is an open issue yet in the normal build, hence "unlikely".

src/VideoManager/SubtitleWriter.cc

@@ -58,7 +58,9 @@ void SubtitleWriter::startCapturingTelemetry(const QString& videoFile)
         QmlObjectListModel* list = grid->columns()->value<QmlObjectListModel*>(colIndex);
         for (int rowIndex = 0; rowIndex < list->count(); rowIndex++) {
             InstrumentValueData* value = list->value<InstrumentValueData*>(rowIndex);
-            _facts += value->fact();
+            if (value->fact()) {
+                _facts += value->fact();
+            }
         }
     }
     grid->deleteLater();