i#3049: clean postcall_cache when drwrap_exit (#3064)

This commit is the supplement for PR #3050.
We also need to clean postcall_cache on drwrap_exit, otherwise
post_callback will not be invoked at re-attach. This is because the registration
of post_callback relies on pre_callback, and the pre_callback checks postcall_cache
before registering the post_callback. The stale data in postcall_cache prevents
post_callback being registered to the hash table.

Issue: #3065, #2157 
Fixes #3049

ext/drwrap/drwrap.c

@@ -981,6 +981,11 @@ drwrap_exit(void)
         !dr_unregister_delete_event(drwrap_fragment_delete))
         ASSERT(false, "failed to unregister in drwrap_exit");
 
+    for (int i = 0; i < POSTCALL_CACHE_SIZE; i++) {
+        postcall_cache[i] = NULL;
+    }
+    postcall_cache_idx = 0;
+
     hashtable_delete(&replace_table);
     hashtable_delete(&replace_native_table);
     hashtable_delete(&wrap_table);