Fix FTP file descriptor leak #3346

Job	Run time
ubuntu-20.04 (non-mavsdk_server, non-superbuild)	4m 12s
Ubuntu/Debian packaging (Ubuntu 20.04)	6m 35s
ubuntu-18.04 (package, non-mavsdk_server)	5m 4s
ubuntu-20.04 (package, non-mavsdk_server)	7m 46s
coverage (ubuntu-20.04)	3m 42s
linux-armv6	4m 11s
linux-armv7	4m 58s
linux-arm64	4m 56s
manylinux2010-x64	7m 32s
fedora-34 (package, non-mavsdk_server)	6m 8s
fedora-35 (package, non-mavsdk_server)	6m 4s
ubuntu-20.04 (check style and docs)	5m 16s
ubuntu-20.04 (mavsdk, hunter)	3m 53s
ubuntu-20.04 (proto check)	12m 53s
ubuntu-20.04 (mavsdk_server, superbuild)	9m 29s
alpine 3.11.6 (musl)	11m 18s
android-arm	7m 41s
android-arm64	6m 28s
android-x86	6m 29s
android-x86_64	6m 24s
linux-armv6-musl	10m 45s
linux-armv7l-musl	10m 28s
linux-arm64-musl	14m 4s
ios	9m 25s
ios_simulator	9m 50s
macOS	12m 34s
macOS-framework	12m 12s
Windows	11m 3s
iOS XCFramework	57s
	3h 42m 17s

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