row_merge_create_index_graph(): Relay the internal state
from dict_create_index_step(). Our caller should free the index
only if it was not copied, added to the cache, and freed.

row_merge_create_index(): Free the index template if it was
not added to the cache. This is a safer variant of the logic
that was introduced in 65070bef in 10.2.

prepare_inplace_alter_table_dict(): Add additional fault injection
to exercise a code path where we have already added an index
to the cache.