Avoid crashing when canonicalize_file_name() fails

I could reproduce this with: grub-probe /dev/sda1

Index: grub/util/getroot.c
===================================================================
--- grub.orig/util/getroot.c	2012-01-30 00:29:30.754018000 -0600
+++ grub/util/getroot.c	2012-01-30 23:31:58.941211000 -0600
@@ -842,7 +931,12 @@
 	{
 	  char *tmp = *cur;
 	  int root, dm;
-	  *cur = canonicalize_file_name (*cur);
+	  *cur = canonicalize_file_name (tmp);
+	  if (*cur == NULL)
+	    {
+	      grub_util_error (_("failed to get canonical path of %s"), tmp);
+	      break;
+	    }
 	  free (tmp);
 	  root = (strcmp (*cur, "/dev/root") == 0);
 	  dm = (strncmp (*cur, "/dev/dm-", sizeof ("/dev/dm-") - 1) == 0);