CONC-812 side-effect on temporal types

Binary-protocol reads of DATE/TIME/DATETIME/TIMESTAMP columns return empty strings.

After CONC-812, mysql_stmt_bind_result sets length_value = 0 for types with negative pack_len. ps_fetch_datetime fills the MYSQL_TIME buffer but never writes *r_param->length back, so the binding reports length 0 after fetch. Connector/C++ then wraps/caches a zero-length buffer and getString() returns "".

In BinRowProtocolCapi (setPosition and cacheCurrentRow), fall back to bind[i].buffer_length (= sizeof(MYSQL_TIME)) when length_value == 0 for temporal buffer types.