Fix rchk errors

[krlmlr]

• Bump vendored cpp11 to 0.5.1, restoring our patches
• Fix other rchk issues
• Fix uninitialized warning

[krlmlr]

Current results:

Function duckdb::RApiTypes::DetectRType(SEXPREC*, bool)
  [UP] unprotected variable names while calling allocating function duckdb::RApiTypes::DetectRType(SEXPREC*, bool) /rchk/packages/build/o2sCSHZS/duckdb/src/types.cpp:159

Function rapi_bind(cpp11::external_pointer<duckdb::RStatement, &(void cpp11::default_deleter<duckdb::RStatement>(duckdb::RStatement*))>, cpp11::r_vector<SEXPREC*>, bool, bool)
  [UP] calling allocating function cpp11::writable::r_vector<SEXPREC*>::push_back(SEXPREC*) with argument allocated using rapi_execute(cpp11::external_pointer<duckdb::RStatement, &(void cpp11::default_deleter<duckdb::RStatement>(duckdb::RStatement*))>, bool, bool) /rchk/packages/build/o2sCSHZS/duckdb/src/statement.cpp:215

Function rapi_execute(cpp11::external_pointer<duckdb::RStatement, &(void cpp11::default_deleter<duckdb::RStatement>(duckdb::RStatement*))>, bool, bool)
  [UP] unprotected variable <unnamed var:   %4 = alloca %struct.SEXPREC*, align 8> while calling allocating function duckdb::unique_ptr<duckdb::QueryResult, std::__1::default_delete<duckdb::QueryResult>, true>::~unique_ptr() /rchk/packages/build/o2sCSHZS/duckdb/src/statement.cpp:382

[krlmlr]

Done now.