PDO: fix broken FetchField function
PDO: fix broken FetchField function

Coding guidelines changes actually altered the function's logic by
replacing an if/elseif block by a non-equivalent switch statement.

Regression introduced by a36f5c4276f7bbf8fc79039a950b0c68d06d2099.

Fixes #121, #122

paulproteus/ADOdb,pyscht/ADOdb,paulproteus/ADOdb,andrewnicols/ADOdb,andrewnicols/ADOdb,ipso/ADOdb,pyscht/ADOdb,ipso/ADOdb