PgBatchConnection may pass strings without a null terminator to libpq APIs

[LordMZTE]

Describe the bug
This code:

drogon/orm_lib/src/postgresql_impl/PgBatchConnection.cc

Line 304 in 8d640ba

cmd->sql_.data(),

may invoke PQsendPrepare with a C-style string that's missing a null terminator.

To Reproduce
Execute an SQL statement with a DbConnection, passing an std::string_view that doesn't have a null terminator in order to call this function:

drogon/orm_lib/inc/drogon/orm/DbClient.h

Lines 252 to 255 in 8d640ba

	internal::SqlBinder operator<<(const std::string_view &sql)
	{
	return internal::SqlBinder(sql.data(), sql.length(), *this, type_);
	}

Expected behavior
We add a null terminator if we can't be sure we already have one.

Desktop (please complete the following information):

• OS: NixOS
• Browser: Firefox
• Version: 1.9.10

Additional context
This gist is my code and log output when I initially encountered this; here, the symptom is that multiple SQL statements are incorrectly combined: https://gist.github.com/LordMZTE/9b707013112b870196ae9f95e4a748f5

[an-tao]

@LordMZTE thanks for your feedback, please check the #2329 out.
BTW, Given the asynchronous interface, the user should ensure that the string_view is valid before the callback is called.