Prevent use of uninitialized variable

Per buildfarm member longfin.

src/backend/storage/lmgr/lmgr.c

@@ -880,6 +880,7 @@ WaitForLockersMultiple(List *locktags, LOCKMODE lockmode, bool progress)
 		holders = lappend(holders,
 						  GetLockConflicts(locktag, lockmode,
 										   progress ? &count : NULL));
+		if (progress)
 			total += count;
 	}