Be more wary about partially-valid LOCALLOCK data in RemoveLocalLock().

RemoveLocalLock() must consider the possibility that LockAcquireExtended()
failed to palloc the initial space for a locallock's lockOwners array.
I had evidently meant to cope with this hazard when the code was originally
written (commit 1785aceb), but missed that
the pfree needed to be protected with an if-test.  Just to make sure things
are left in a clean state, reset numLockOwners as well.

Per low-memory testing by Andreas Seltenreich.  Back-patch to all supported
branches.