Avoid panic in handleValuePostings (dgraph-io#5652)

Fixes: DGRAPH-1608

srcFn.n should be equal to len(q.UidList.Uids) for implementation DivideAndRule and
calculate to work correctly. But we have seen some panics while forming DataKey in
calculate(). panic is of the form "index out of range [4] with length 1". We have tried to
reproduce this but couldn't find any edge case which will result in this panic. So for now
we are just logging when srcFn.n != len(q.UidList.Uids) and returning error when this
happens.

worker/task.go

@@ -358,6 +358,15 @@ func (qs *queryState) handleValuePostings(ctx context.Context, args funcArgs) er
 		return nil
 	}
 
+	// srcFn.n should be equal to len(q.UidList.Uids) for below implementation(DivideAndRule and
+	// calculate) to work correctly. But we have seen some panics while forming DataKey in
+	// calculate(). panic is of the form "index out of range [4] with length 1". Hence return error
+	// from here when srcFn.n != len(q.UidList.Uids).
+	if srcFn.n != len(q.UidList.Uids) {
+		return errors.Errorf("srcFn.n: %d is not equal to len(q.UidList.Uids): %d, srcFn: %+v in "+
+			"handleValuePostings", srcFn.n, len(q.UidList.GetUids()), srcFn)
+	}
+
 	// This function has small boilerplate as handleUidPostings, around how the code gets
 	// concurrently executed. I didn't see much value in trying to separate it out, because the core
 	// logic constitutes most of the code volume here.