shared/archive: Fix crash on nil tracker

Signed-off-by: Stéphane Graber <stgraber@stgraber.org>

shared/archive/archive.go

@@ -188,7 +188,7 @@ func Unpack(file string, path string, blockBackend bool, maxMemory int64, tracke
 		}
 	} else if strings.HasPrefix(extension, ".squashfs") {
 		// Progress tracking with squashfs doesn't work as it needs to seek the file.
-		if tracker.Handler != nil {
+		if tracker != nil && tracker.Handler != nil {
 			tracker.Handler(0, 0)
 		}