tests: don't panic after failing to run Ignition
tests: don't panic after failing to run Ignition

This commit adds a nil check for cmd.Process.Pid to prevent
a nil pointer dereference when running Ignition fails.

Signed-off-by: Kamil Domański <ab239c5a26a103f02214f1ae199f6dad0108e000@domanski.co>

coreos/ignition,coreos/ignition,coreos/ignition