mirror: mirror --set/--unset fails when --path is not used

Before swupd_init if --path is not used, path_prefix is NULL. Calling
set_default_path_prefix() to force it back to correct value.

Signed-off-by: Otavio Pontes <otavio.pontes@intel.com>

src/mirror.c

@@ -364,6 +364,9 @@ enum swupd_code mirror_main(int argc, char **argv)
 		print_help();
 		return SWUPD_INVALID_OPTION;
 	}
+	if (!globals.path_prefix) {
+		set_default_path_prefix();
+	}
 
 	progress_init_steps("mirror", steps_in_mirror);