kdb+ plugin uses unsafe C++ code

[FlyingOE]

Assigning the result of str.c_str() to a pointer to be used later is against the C++ standard and is not safe. For example: https://github.com/dolphindb/DolphinDBPlugin/blob/30535a5e76a2ccbfa9820de396313c8cf9d713b5/kdb/src/kdb.cpp#L61