ogsf: fix crash when unloading areas

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@56497 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/ogsf/Gv3.c

@@ -145,6 +145,7 @@ geoline *Gv_load_vect(const char *grassname, int *nlines)
 
 	}
 
+	gln->cats = NULL;
 	gln->next = (geoline *) G_malloc(sizeof(geoline));	/* G_fatal_error */
 	if (!gln->next) {
 	    return (NULL);