emu: Make grub_free(NULL) safe

The grub_free() implementation in grub-core/kern/mm.c safely handles
NULL pointers, and code at many places depends on this. We don't know
that the same is true on all host OSes, so we need to handle the same
behavior in grub-emu's implementation.

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

grub-core/kern/emu/mm.c

@@ -60,6 +60,7 @@ grub_zalloc (grub_size_t size)
 void
 grub_free (void *ptr)
 {
+  if (ptr)
     free (ptr);
 }