Mandelbrot: Fix 'new[]' array freed with 'delete'

fRenderBuffer is allocated by new[] in MessageReceived(),
buf freed by delete.
Pointed out by LGTM.

Change-Id: I06efc1c8075a421e7f2983e8a0d817a1bf43b026
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1880
Reviewed-by: Axel Dörfler <[email protected]>

src/apps/mandelbrot/FractalEngine.cpp

@@ -111,7 +111,7 @@ void FractalEngine::MessageReceived(BMessage* msg)
 		fResizing = true;
 		StopRender();
 
-		delete fRenderBuffer;
+		delete[] fRenderBuffer;
 		fRenderBuffer = NULL;
 
 		fWidth = msg->GetUInt16("width", 320);