pgsql: fix fetchField() parameter naming
pgsql: fix fetchField() parameter naming

Regression from 9f6f327b9d5b29d14565fa0ab4308354bf050d2d.

fetchField()'s parameter was renamed from $off to $fieldOffset, but the
change was not reflected in the method's body.

Fixes #752

ipso/ADOdb,ipso/ADOdb