trx_undo_prev_version_build() should confirm existence of inherited (not-own) external pages.

Bug #14676084 : ROW_UNDO_MOD_UPD_DEL_SEC() DOESN'T NEED UNDO_ROW AND UNDO_EXT INITIALIZED
 mtr script could hit the assertion error !bpage->file_page_was_freed using this path.
 So, also fixed

rb://1337 approved by Marko Makela.