osdep/unix/getroot: Fix potential underflow

The entry_len is initialized in grub_find_root_devices_from_mountinfo()
to 0 before the while loop iterates through /proc/self/mountinfo. If the
file is empty or contains only invalid entries entry_len remains
0 causing entry_len - 1 in the subsequent for loop initialization
to underflow. To prevent this add a check to ensure entry_len > 0 before
entering the for loop.

Fixes: CID 473877

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>

grub-core/osdep/linux/getroot.c

@@ -484,6 +484,9 @@ again:
 	}
     }
 
+  if (!entry_len)
+    goto out;
+
   /* Now scan visible mounts for the ones we're interested in.  */
   for (i = entry_len - 1; i >= 0; i--)
     {