kern/efi/mm: Fix possible NULL pointer dereference

The model of grub_efi_get_memory_map() is that if memory_map is NULL,
then the purpose is to discover how much memory should be allocated to
it for the subsequent call.

The problem here is that with grub_efi_is_finished set to 1, there is no
check at all that the function is being called with a non-NULL memory_map.

While this MAY be true, we shouldn't assume it.

The solution to this is to behave as expected, and if memory_map is NULL,
then don't try to use it and allow memory_map_size to be filled in, and
return 0 as is done later in the code if the buffer is too small (or NULL).

Additionally, drop unneeded ret = 1.

Fixes: CID 96632

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

grub-core/kern/efi/mm.c

@@ -339,15 +339,24 @@ grub_efi_get_memory_map (grub_efi_uintn_t *memory_map_size,
   if (grub_efi_is_finished)
     {
       int ret = 1;
-      if (*memory_map_size < finish_mmap_size)
+
+      if (memory_map != NULL)
 	{
-	  grub_memcpy (memory_map, finish_mmap_buf, *memory_map_size);
+	  if (*memory_map_size < finish_mmap_size)
-	  ret = 0;
+	    {
+	      grub_memcpy (memory_map, finish_mmap_buf, *memory_map_size);
+	      ret = 0;
+	    }
+          else
+	    grub_memcpy (memory_map, finish_mmap_buf, finish_mmap_size);
 	}
       else
 	{
-	  grub_memcpy (memory_map, finish_mmap_buf, finish_mmap_size);
+	  /*
-	  ret = 1;
+	   * Incomplete, no buffer to copy into, same as
+	   * GRUB_EFI_BUFFER_TOO_SMALL below.
+	   */
+	  ret = 0;
 	}
       *memory_map_size = finish_mmap_size;
       if (map_key)