Fix concurrent issue in SingleSignOutFilter#doFilter method

[taydy]

Fix concurrency issues in SingleSignOutFilter#doFilter method.

In the SingleSignOutFilter#doFilter method, synchronization is added to prevent concurrent execution and ensure thread safety.
Before the repair, if SingleSignOutFilter#init is not executed for initialization, when multiple threads execute concurrently, one thread may be executing the HANDLER.init() method, the safeParameters in SingleSignOutHandler have not yet been initialized, and other threads have begun executing HANDLER.process(request, response), which will causes the program to throw a NullPointerException.

[taydy]

@mmoayyed Could you please take a look at this issue?