We have quite a few assertions
	ut_a(m_prebuilt->trx == thd_to_trx(ha_thd()));
in low-level functions.
These had better be debug assertions for performance reasons.
It should suffice to check that condition in the less frequently invoked
ha_innobase::change_active_index().

convert_search_mode_to_innobase(): Return whether the mode is
unsupported, and optionally update ha_innobase::m_last_match_mode.

ha_innobase::index_read(): Only branch on find_flag once, and
simplify the error handling after invoking row_search_mvcc().

ha_innobase::rnd_pos(): Remove an assertion that is duplicating one
in ha_innobase::index_read(), which we are calling unconditionally.

ha_innobase::records_in_range(): Check only once whether
min_key, max_key are null pointers.

row_sel_convert_mysql_key_to_innobase(): Declare all parameters
except the conversion buffer pointer (buf) to be nonnull.

Reviewed by: Debarun Banerjee