Don't cast partially destroyed object, fixes UBSAN report:

```
/home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/language/highlighting/codehighlighting.cpp:612:21: runtime error: downcast of address 0x608000240320 which does not point to an object of type 'DocumentChangeTracker'
0x608000240320: note: object is of type 'QObject'
 14 01 80 2f  e0 84 05 4b 5d 7f 00 00  a0 03 24 00 80 60 00 00  10 ee 11 00 20 60 00 00  01 be be be
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'QObject'
    #0 0x7f5d566f5920 in KDevelop::CodeHighlighting::trackerDestroyed(QObject*) /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/language/highlighting/codehighlighting.cpp:612
```