don't set layer to 0 for layers without table

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@41951 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/vector/Vlib/field.c

@@ -505,7 +505,7 @@ int Vect_get_field_number(const struct Map_info *Map, const char *field)
     if (fi)
 	return fi->number;
 
-    return 0;
+    return atoi(field);
 }
 
 /*!