ha_innobase::final_drop_index(): If row_merge_drop_table() fails, clear
the to_be_dropped flags.  This was the error fixed in this commit; the rest
is just additional safety.

ha_innobase::final_drop_index(): After dropping the flagged indexes,
assert that none of the remaining indexes are flagged to_be_dropped.

ha_innobase::prepare_drop_index(): Assert that no index has been flagged
for deletion.  When checking foreign key constraints, simply traverse the
list of indexes and check if any of the indexes that were just flagged
to_be_dropped.  On error, clear the to_be_dropped flags with simple list
traversal.