Set a default value for slot in SharedMutex.h

Set a default value for slot in SharedMutex.h

Summary:
`SharedMutexImpl::lockSharedImpl` has a potentially uninitialized access:

  Assume state = 0
  canAlreadyDefer = (state & kMayDefer) != 0 ==> false
  aboveDeferThreshold = (state & kHasS) >= (kNumSharedToStartDeferring - 1) * kIncrHasS ==> false

  if (canAlreadyDefer || (aboveDeferThreshold && !drainInProgress)) ==> false

  line:1452: gotSlot(slot)->compare_exchange_strong(...) uses slot uninitialized

Reviewed By: Orvid

Differential Revision: D3933638

fbshipit-source-id: 0fbce5c00b8b1f34e50c302cb88def97853c5afe