patch 8.2.0825: def_function() may return pointer that was freed

Problem:    def_function() may return pointer that was freed.
Solution:   Set "fp" to NULL after freeing it.

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    825,
 /**/
     824,
 /**/