backport: bitcoin-core/gui#693: Fix segfault when shutdown during wallet open #1922

Job	Run time
Build container / Build container	20s
x86_64-pc-linux-gnu / Build depends	57s
arm-linux-gnueabihf / Build depends	56s
x86_64-pc-linux-gnu_nowallet / Build depends	56s
x86_64-pc-linux-gnu_multiprocess / Build depends	1m 7s
x86_64-w64-mingw32 / Build depends	57s
x86_64-apple-darwin / Build depends	1m 1s
linux64_nowallet-build / Build source	6m 54s
linux64_ubsan-build / Build source	8m 5s
linux64_sqlite-build / Build source	7m 5s
linux64-build / Build source	6m 19s
linux64_fuzz-build / Build source	17m 20s
arm-linux-build / Build source	4m 43s
win64-build / Build source	11m 22s
mac-build / Build source	4m 44s
linux64_multiprocess-build / Build source	36m 47s
linux64_tsan-build / Build source	12m 59s
linux64-test / Test source	34m 59s
linux64_nowallet-test / Test source	13m 30s
linux64_sqlite-test / Test source	31m 14s
linux64_ubsan-test / Test source	32m 28s
	3h 54m 43s

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