zfsinfo: Correct a check for error allocating memory

While arguably the check for grub_errno is correct, we should really be
checking the return value from the function since it is always possible
that grub_errno was set elsewhere, making this code behave incorrectly.

Fixes: CID 73668

Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/fs/zfs/zfsinfo.c

@@ -358,8 +358,8 @@ grub_cmd_zfs_bootfs (grub_command_t cmd __attribute__ ((unused)), int argc,
     return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("one argument expected"));
 
   devname = grub_file_get_device_name (args[0]);
-  if (grub_errno)
-    return grub_errno;
+  if (devname == NULL)
+    return GRUB_ERR_OUT_OF_MEMORY;
 
   dev = grub_device_open (devname);
   grub_free (devname);