Fix double-release of spinlock

Fix double-release of spinlock

Commit 9d9b9d46f3 added spinlocks to protect the fields in ProcSignal
flags, but in EmitProcSignalBarrier(), the spinlock was released
twice. With most spinlock implementations, releasing a lock that's not
held is not easy to notice, because most of the time it does nothing,
but if the spinlock was concurrently acquired by another process, it
could lead to more serious issues. Fortunately, with the
--disable-spinlocks emulation implementation, it caused more visible
failures.

In the passing, fix a type in comment and add an assertion that the
procNumber passed to SendProcSignal looks valid.

Discussion: https://www.postgresql.org/message-id/b8ce284c-18a2-4a79-afd3-1991a2e7d246@iki.fi