Bug #45058 init_available_charsets uses double checked locking

A client doing multiple mysql_library_init() and
mysql_library_end() calls over the lifetime of the process may
experience lost character set data, potentially even a
SIGSEGV.

This patch reinstates the reloading of character set data when
a mysql_library_init() is done after a mysql_library_end().

libmysql/libmysql.c

@@ -211,6 +211,7 @@ void STDCALL mysql_server_end()
   }
   else
   {
+    free_charsets();
     mysql_thread_end();
   }