Allow WaitLatch() to be used without a latch.
Due to flaws in commit
3347c982bab, using WaitLatch() without
WL_LATCH_SET could cause an assertion failure or crash. Repair.
While here, also add a check that the latch we're switching to belongs
to this backend, when changing from one latch to another.
Discussion: https://postgr.es/m/CA%2BhUKGK1607VmtrDUHQXrsooU%3Dap4g4R2yaoByWOOA3m8xevUQ%40mail.gmail.com