Fix #80783: PDO ODBC truncates BLOB records at every 256th byte

It is not guaranteed, that the driver inserts only a single NUL byte at
the end of the buffer. Apparently, there is no wa

Fix #80783: PDO ODBC truncates BLOB records at every 256th byte

It is not guaranteed, that the driver inserts only a single NUL byte at
the end of the buffer. Apparently, there is no way to find out the
actual data length in the buffer after calling `SQLGetData()`, so we
adjust after the next `SQLGetData()` call.

We also prevent PDO::ODBC_ATTR_ASSUME_UTF8 from fetching garbage, by
fetching all chunks with the same C type.

Closes GH-6716.

show more ...