cl/rpc: use count param for blob sidecar slice preallocation

[VolodymyrBg]

The count parameter in sendBlobsSidecar

erigon/cl/rpc/rpc.go

Lines 99 to 114 in d2f7395

	func (b *BeaconRpcP2P) sendBlobsSidecar(ctx context.Context, topic string, reqData []byte, count uint64) ([]*cltypes.BlobSidecar, string, error) {
	responses, pid, err := b.sendRequest(ctx, topic, reqData)
	if err != nil {
	return nil, pid, err
	}
	responsePacket := []*cltypes.BlobSidecar{}
	for _, data := range responses {
	responseChunk := &cltypes.BlobSidecar{}
	if err := responseChunk.DecodeSSZ(data.raw, int(data.version)); err != nil {
	return nil, pid, err
	}
	responsePacket = append(responsePacket, responseChunk)
	}
	return responsePacket, pid, nil

was accepted but never used - both callers correctly compute the expected max number of blob sidecars, but the result slice was initialized as an empty literal []*BlobSidecar{} ignoring this value entirely.

Use make([]*cltypes.BlobSidecar, 0, count) to preallocate the response slice, avoiding unnecessary grow-and-copy cycles during append.

[AskAlexSharov]

count coming from outside of program. maybe from User/Attacker. Means can't use it as a parameter of pre-allocating - because it's attack-vector.