MDEV-4815 - allow multiple mysql_server_init() / mysql_server_end() in the same process, for embedded library.

- Reset  static variables that are used to signal "init done"  for DBUG, in dbug_end()
- Set string server variables to NULL after memory for the value is freed - avoids double free()
- fix DBUG_ASSERTs that happened during reinitialization.

dbug/dbug.c

@@ -1642,6 +1642,7 @@ void _db_end_()
 
   cs->stack= &init_settings;
   FreeState(cs, 0);
+  init_done= 0;
 }
 
 

mysys/waiting_threads.c

@@ -475,6 +475,7 @@ void wt_end()
   my_atomic_rwlock_destroy(&cycle_stats_lock);
   my_atomic_rwlock_destroy(&success_stats_lock);
   my_atomic_rwlock_destroy(&wait_stats_lock);
+  reshash.alloc.constructor= NULL;
   wt_init_done= 0;
   DBUG_VOID_RETURN;
 }

sql/sys_vars.h

@@ -427,7 +427,10 @@ public:
   void cleanup()
   {
     if (flags & ALLOCATED)
+    {
       my_free(global_var(char*));
+      global_var(char *)= NULL;
+    }
     flags&= ~ALLOCATED;
   }
   static bool do_string_check(THD *thd, set_var *var, CHARSET_INFO *charset)

storage/xtradb/os/os0file.c

@@ -3773,6 +3773,10 @@ os_aio_free(void)
 	ut_free(os_aio_segment_wait_events);
 	os_aio_segment_wait_events = 0;
 	os_aio_n_segments = 0;
+#ifdef _WIN32
+	completion_port = 0;
+	read_completion_port = 0;
+#endif
 }
 
 #ifdef WIN_ASYNC_IO