APPS/pkcs8: fix case where infile and outfile are the same #2200

Job	Run time
check_docs	57s
check_update	1m 23s
basic_clang	6m 16s
basic_gcc	5m 39s
check-ansi	2m 36s
minimal	2m 34s
no-deprecated	5m 25s
self-hosted	0s
address_ub_sanitizer	10m 38s
enable_non-default_options	4m 43s
external-test-cf-quiche	3m 33s
external-test-pyca (1.51.0, 3.9)	5m 54s
external-tests	11m 20s
full_featured	5m 53s
fuzz_tests	2m 59s
legacy	4m 36s
memory_sanitizer	17m 40s
no-legacy	5m 26s
non-caching	12m 39s
no-shared-macos (macos-13)	12m 15s
no-shared-ubuntu	4m 39s
no-shared-macos (macos-14)	3m 36s
out-of-readonly-source-and-install-macos (macos-13)	11m 26s
out-of-readonly-source-and-install-macos (macos-14)	6m 2s
out-of-readonly-source-and-install-ubuntu	7m 26s
threads_sanitizer	5m 3s
	2h 40m 38s

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