depmod: Fix crash in previous commit if root is not set

[This fixes http://build.gnome.org/continuous/buildmaster/builds/2015/02/25/31/build/output.txt ]

The variable we're reading here is "root", not "optarg" which is only
valid inside the getopt call.
diff --git a/tools/depmod.c b/tools/depmod.c
index a4a7479..18aab5d 100644
--- a/tools/depmod.c
+++ b/tools/depmod.c
@@ -2484,7 +2484,7 @@
 	}
 
 	if (root)
-		root = path_make_absolute_cwd(optarg);
+		root = path_make_absolute_cwd(root);
 
 	cfg.dirnamelen = snprintf(cfg.dirname, PATH_MAX,
 				  "%s/lib/modules/%s",