dict_drop_index_tree(): Even if SYS_INDEXES.PAGE contains the
special value FIL_NULL, the tablespace identified by SYS_INDEXES.SPACE
may exist and may need to be dropped. This would definitely be the case
if the server had been killed right after a FILE_CREATE record was
persistently written during CREATE TABLE, but before the transaction
was committed.

btr_free_if_exists(): Simplify the interface, to avoid repeated
tablespace lookup.

One more scenario is known to be broken: If the server is killed
during DROP TABLE (or table-rebuilding ALTER TABLE) right after a
FILE_DELETE record has been persistently written but before the
file was deleted, then we could end up recovering no tablespace
at all, and failing to delete the file, in either of fil_name_process()
or dict_drop_index_tree().

Thanks to Elena Stepanova for providing "rr replay" and data directories
of these scenarios.