Fix Mantis issue#61:

In row_undo_ins_parse_undo_rec(): if we find that a table has no indexes
(dict_table_get_first_index() returns NULL) do not try to call
trx_undo_rec_get_row_ref() with a NULL pointer because that would lead to
a crash. Instead, print a warning and set node->table to NULL just like
it is done if the .ibd file is missing.

Approved by:	Heikki (via IM)