Fix FTP file descriptor leak #3343

Job	Run time
alpine 3.11.6 (musl)	31m 26s
coverage (ubuntu-20.04)	5m 45s
ubuntu-18.04 (package, non-mavsdk_server)	4m 58s
ubuntu-20.04 (package, non-mavsdk_server)	5m 31s
Ubuntu/Debian packaging (Ubuntu 20.04)	10m 52s
android-arm	15m 13s
android-arm64	15m 21s
android-x86	15m 39s
android-x86_64	16m 13s
linux-armv6-musl	29m 54s
linux-armv7l-musl	26m 11s
linux-arm64-musl	26m 56s
linux-armv6	5m 2s
linux-armv7	4m 56s
linux-arm64	5m 6s
manylinux2010-x64	21m 1s
ios	30m 25s
ios_simulator	29m 31s
macOS	30m 51s
macOS-framework	33m 51s
fedora-34 (package, non-mavsdk_server)	6m 37s
fedora-35 (package, non-mavsdk_server)	5m 7s
ubuntu-20.04 (check style and docs)	55s
ubuntu-20.04 (mavsdk, hunter)	8m 6s
ubuntu-20.04 (non-mavsdk_server, non-superbuild)	4m 55s
ubuntu-20.04 (proto check)	12m 10s
ubuntu-20.04 (mavsdk_server, superbuild)	27m 54s
Windows	45m 50s
iOS XCFramework	1m 9s
	7h 57m 25s

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