use .IsSet() instead if .Get() != NULL

Change-Id: Ia2b7a719fd398e78cc3b11d4f7b02cb81179f65f
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3488
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

app_server memory management fixes: use BReference

Use BReference for more automated reference counting in app_server,
fixing some use-after-free and other problems.

Extracted from https://review.haiku-os.org/c/haiku/+/2695

Change-Id: I141bb248229405896b29feff3338447f7257b0b4
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3175
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

Fix #11775.

BitmapDrawingEngine:
- Check if fBitmap is NULL before releasing its reference. Since this is
the case when a BitmapDrawingEngine is instantiated, this would lead to
an app_server crash upon any attempt to make use of one.

Forgot to reset fBitmap to NULL.

Thanks to Rene for reviewing.

Call ReleaseReference when a BReference is not needed

Reduces complexity and is actualñly more readable.

app_server: fix misuse of BReferenceable.

Fixes #9989.

app_server: Cleanup of some file locations

Fix #11775.

BitmapDrawingEngine:
- Check if fBitmap is NULL before releasing its reference. Since this is
the case when a BitmapDrawingEngine is instantiated, this would lead to
an app_server crash upon any attempt to make use of one.

Forgot to reset fBitmap to NULL.

Thanks to Rene for reviewing.

Call ReleaseReference when a BReference is not needed

Reduces complexity and is actualñly more readable.

app_server: fix misuse of BReferenceable.

Fixes #9989.

app_server: Cleanup of some file locations