Fix error in UI_butstore_ API

If the `uiButStore` data was freed after the buttons/blocks were updated
from previous instances (see `UI_block_update_from_old()`), e.g. by
delaying that to the "afterfuncs" (`ui_apply_but_funcs_after()`), the
data would get lost. As result, the button pointers that the API is
supposed to keep valid would point to freed memory.

This wasn't an issue so far since the API didn't happen to be used this
way. That changes with the next commit.

source/blender/editors/interface/interface_utils.c

@@ -647,6 +647,7 @@ void UI_butstore_free(uiBlock *block, uiButStore *bs_handle)
   }
 
   BLI_freelistN(&bs_handle->items);
+  BLI_assert(BLI_findindex(&block->butstore, bs_handle) != -1);
   BLI_remlink(&block->butstore, bs_handle);
 
   MEM_freeN(bs_handle);
@@ -747,8 +748,7 @@ void UI_butstore_update(uiBlock *block)
   /* move this list to the new block */
   if (block->oldblock) {
     if (block->oldblock->butstore.first) {
-      block->butstore = block->oldblock->butstore;
-      BLI_listbase_clear(&block->oldblock->butstore);
+      BLI_movelisttolist(&block->butstore, &block->oldblock->butstore);
     }
   }