efi_loader: avoid NULL dereference in efi_get_memory_map()

We should only dereference parameter memory_map_size after checking that
it is valid.

Fixes: 8e835554b36b ("efi_loader: check parameters of GetMemoryMap")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>

lib/efi_loader/efi_memory.c

@@ -457,11 +457,13 @@ efi_status_t efi_get_memory_map(efi_uintn_t *memory_map_size,
 	efi_uintn_t map_size = 0;
 	int map_entries = 0;
 	struct list_head *lhandle;
-	efi_uintn_t provided_map_size = *memory_map_size;
+	efi_uintn_t provided_map_size;
 
 	if (!memory_map_size)
 		return EFI_INVALID_PARAMETER;
 
+	provided_map_size = *memory_map_size;
+
 	list_for_each(lhandle, &efi_mem)
 		map_entries++;