lib/vector/Vlib: Initialize variable before using it in write_sfa mod…

…ule (OSGeo#4696)

lib/vector/Vlib/write_sfa.c

@@ -339,6 +339,7 @@ void V2__add_line_to_topo_sfa(struct Map_info *Map, int line,
     G_debug(3, "V2__add_line_to_topo_sfa(): line = %d npoints = %d", line,
             points->n_points);
 
+    first = TRUE;
     plus = &(Map->plus);
     Line = plus->Line[line];
     type = Line->type;