add a debug statement and a comment about its error handling

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@50459 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/vector/Vlib/header.c

@@ -615,7 +615,15 @@ const char *Vect_get_proj_name(const struct Map_info *Map)
     case PROJECTION_LL:
     case PROJECTION_SP:
 	return G__projection_name(n);
+    default:
+	G_debug(1, "Vect_get_proj_name(): "
+		   "Vect_get_proj() returned an invalid result (%d)", n);
+	break;
     }
+
+    /* Vect_get_proj() didn't return a useful result,
+       fallback to G_database_projection_name() */
+    /* (is this behavior desirable?) */
     if (!lookup(PROJECTION_FILE, "name", name, sizeof(name)))
 	strcpy(name, _("Unknown projection"));