tiledb_attr raises std::bad_alloc with variable length

[johnkerl]

Fixes #828

[cgiachalis]

It looks now that the sanitation is in two places.

[cgiachalis]

By removing this line fixes the issue:

TileDB-R/src/libtiledb.cpp

Line 1539 in 2e7e171

attr->set_cell_val_num(static_cast<uint64_t>(ncells));

Then few lines down -> sanitises if need it and then assigns the cell value

TileDB-R/src/libtiledb.cpp

Lines 1562 to 1564 in 2e7e171

	uint64_t num =
	(ncells == R_NaInt) ? TILEDB_VAR_NUM : static_cast<uint64_t>(ncells);
	attr->set_cell_val_num(num);

[johnkerl]

@mojaveazure ping 🙏