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.

src/backend/storage/lmgr/lock.c

@@ -753,7 +753,7 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		locallock->numLockOwners = 0;
 		locallock->maxLockOwners = 8;
 		locallock->holdsStrongLockCount = FALSE;
-		locallock->lockOwners = NULL;
+		locallock->lockOwners = NULL;	/* in case next line fails */
 		locallock->lockOwners = (LOCALLOCKOWNER *)
 			MemoryContextAlloc(TopMemoryContext,
 						  locallock->maxLockOwners * sizeof(LOCALLOCKOWNER));
@@ -1222,7 +1222,9 @@ RemoveLocalLock(LOCALLOCK *locallock)
 		if (locallock->lockOwners[i].owner != NULL)
 			ResourceOwnerForgetLock(locallock->lockOwners[i].owner, locallock);
 	}
-	pfree(locallock->lockOwners);
+	locallock->numLockOwners = 0;
+	if (locallock->lockOwners != NULL)
+		pfree(locallock->lockOwners);
 	locallock->lockOwners = NULL;
 
 	if (locallock->holdsStrongLockCount)