qt: Make sure splash screen is freed on AppInitMain fail

Summary: Backport of core PR12374

Test Plan:
Launch bitcoin-qt and closes the splash screen exactly at the end of the
AppInitMain process (refer to core issue #12372)

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2194