PG: fix retrieval of SRID on a table without SRID constraint, and when the datasource is opened with ' tables=fake' (#5131)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26508 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx