Fix for bug #2272: shp2pgsql-gui crashes when given filenames with dots.

Fix for bug #2272: shp2pgsql-gui crashes when given filenames with dots.

This was a combination of two bugs: firstly, we weren't initialising the
state variable field_names to NULL, and so if the variable happened to
be filled with junk and we were unable to import a shape file, we would
end up freeing a random pointer.

Secondly, shapelib tries to be clever and strip off any file extension
by searching for a "." character from the end of the filename, hence
causing the truncation of the filename. Resolve this by leaving the
.shp/.dbf extension from the file selector in the filename string so
that shapelib does the right thing.

git-svn-id: http://svn.osgeo.org/postgis/trunk@11292 b70326c6-7e19-0410-871a-916f4a2858ee