qt: Make sure splash screen is freed on AppInitMain fail
The `splashFinished` event was never sent if AppInitMain fails,
causing the splash screen to stick around, causing problems
This bug has existed for a while but is now trigging potential crashed
because the splash screen subscribes to wallet events.
Meant to fix #12372.
Signed-off-by: Wladimir J. van der Laan <firstname.lastname@example.org>