Be paranoid about the values sent to upgrade

The docs aren't entirely clear on the order of calls for upgrades so
this is mostly just in case old_priv ever happens to not be what load
returned in priv.

c_src/jiffy.c

@@ -42,7 +42,8 @@ reload(ErlNifEnv* env, void** priv, ERL_NIF_TERM info)
 static int
 upgrade(ErlNifEnv* env, void** priv, void** old_priv, ERL_NIF_TERM info)
 {
-    enif_free(*old_priv);
+    if(*old_priv != NULL)
+        enif_free(*old_priv);
     return load(env, priv, info);
 }