bufmgr: Avoid spurious compiler warning after fcb9c977aa5

bufmgr: Avoid spurious compiler warning after fcb9c977aa5

Some compilers, e.g. gcc with -Og or -O1, warn about the wait_event in
BufferLockAcquire() possibly being uninitialized. That can't actually happen,
as the switch() covers all legal lock mode values, but we still need to
silence the warning.  We could add a default:, but we'd like to get a warning
if we were to get a new lock mode in the future.  So just initialize
wait_event to 0.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/934395.1768518154@sss.pgh.pa.us