Automatically convert SRID<0 to SRID=0 in topology.topology

[strk]

I just found that the topology built by the testsuite (load_topology.sql) succeeds in writing a -1 value in topology.topology.srid field.

This, in turn, can confuse clients: ​http://hub.qgis.org/issues/12802

As we gently convert -1 to 0 in other cases, I think it would be nice to do it also in the topology.topology.srid case.

Since people might already have -1's in their db, part of this would be converting existing values, updating the table.

[robe]

Milestone renamed