mender-artifact panic when getting from ssh (snapshot)

When using mender-artifact to generate the artifact from a remote device (ssh) the OS can prompt for the SSH password. If the user wants to kill the process there (SIGINT), mender-artifact will raise a panic.

How to reproduce:

• Start a mender-artifact write command from a device supporting mender snapshot feature:
  • mender-artifact write rootfs-image -f ssh://pi@10.20.33.251 -n dump -o dump2.mender -t raspberrypi
• When asked for the password, cancel using SIGINT signal
  • pi@10.20.33.251's password: (Ctrl+C)

Error trace:

$ mender-artifact write rootfs-image -f ssh://pi@10.20.33.251 -n dump -o dump2.mender -t raspberrypi
pi@10.20.33.251's password: 
panic: send on closed channel

goroutine 5 [running]:
os/signal.process(0x719d80, 0xc00015c000)
	/snap/go/4286/src/os/signal/signal.go:237 +0x225
os/signal.loop()
	/snap/go/4286/src/os/signal/signal_unix.go:23 +0x52
created by os/signal.init.0
	/snap/go/4286/src/os/signal/signal_unix.go:29 +0x41