r.out.png: reset file pointer to NULL after fclose (OSGeo#4220)

This patch continues the work from 917ba58. It's a good
practice to immediately reset the file pointer once we
do fclose on it, as it prevents using/closing descriptor
allocated to another file in the future execution paths.

Signed-off-by: Mohan Yelugoti <ymdatta.work@gmail.com>

raster/r.out.png/main.c

@@ -369,8 +369,10 @@ int main(int argc, char *argv[])
     /* G_free (info_ptr); */
     png_destroy_write_struct(&png_ptr, &info_ptr); /* al 11/2000 */
 
-    if (fp)
+    if (fp) {
         fclose(fp);
+        fp = NULL;
+    }
 
     if (wld_flag->answer) {
         if (do_stdout)