Check for null values in the PostGIS results and don't add attributes

with null values to the feature rather than adding a value based on
decoding a buffer full of undefined data.

plugins/input/postgis/postgisfs.cpp

@@ -105,6 +105,9 @@ feature_ptr postgis_featureset::next()
         for (unsigned pos=1;pos<num_attrs_+1;++pos)
         {
            std::string name = rs_->getFieldName(pos);
+
+           if (!rs_->isNull(pos))
+           {
               const char* buf=rs_->getValue(pos);
               int oid = rs_->getTypeOID(pos);
            
@@ -158,6 +161,7 @@ feature_ptr postgis_featureset::next()
 #endif
               }
            }
+        }
         ++count_;   
         return feature;
     }

plugins/input/postgis/resultset.hpp

@@ -122,6 +122,11 @@ public:
 	return 0;
     }
 
+    bool isNull(int index) const
+    {
+	return PQgetisnull(res_,pos_,index);
+    }
+    
     const char* getValue(int index) const
     {
 	return PQgetvalue(res_,pos_,index);