ps.map: fix uninitialized pointer found by coverity scan (merge from trunk, https://trac.osgeo.org/grass/changeset/62479)

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@62480 15284696-431f-4ddb-bdfa-cd5b030d7da7

ps/ps.map/ps_vpoints.c

@@ -48,6 +48,7 @@ int PS_vpoints_plot(struct Map_info *P_map, int vec)
     PSCOLOR color;
 
     cv_rgb = NULL;
+    Symb = NULL;
 
     /* Create vector array if required */
     if (vector.layer[vec].cats != NULL || vector.layer[vec].where != NULL) {